It is a default private IP for various NAT/network devices such as routers. In order to resolve the issue of shortage of IP to be used on the Internet, these IP addresses were devised. RFC 1918 defines the ranges of the private IP.

An IP, where IP stands for Internet Protocol, is a numeric identification that is assigned to all the devices or machines, which are a part of any computer network. All the devices, which are connected to a network, have a unique IP. As we have said earlier, these addresses are numeric representations, which are divided into four parts, and each part is separated by a period (dot) between them. For instance, an IP normally looks like 192.168.1.1.

One series of these numbers is classified as private IP. Generally, this set of numbers range from 192.168.0.0 to 192.168.255.255. Each of these numbers is retained and unique as compared to other IP addresses. The word unique here is used in the sense that these numbers are unique within the network they belong. To make this clear let us take an example of your college or office computer, which is assigned an IP 192.168.1.1 in network and you, will have the same IP address in your home network as well and it won’t create any conflicting issue.

NAT (Network Address Translation) and Private Address Space

With the evolution of Internet, there started to be a shortage of IP addresses simply due to the way through which these are allocated. For a quick fix of this problem, NAT devices are used. For instance, if you have more than one computer in your home and all of them are connected to a NAT device say router, that router assigned a unique private Ip to each computer.

The NAT devices, such as router or firewall, modify the information that your computer send to the Internet, hence in turn they get a normal address that can be used on the web. Another important feature of the NAT device is that they are accountable to route the information entering the network to the right computer in your network.

Default IP

192.168.1.1 is a default IP address used by various NAT devices in private networks such as routers. Some other default IP addresses include 192.168.2.1 and 192.168.0.1. As we have said in the beginning that the ranges of these IP is defined by RFC 1918. The Internet Assigned Numbers Authority (IANA) is responsible for keeping these and other numbers reserved within their particular ranges for private network use.

There are so many Nat devices or network devices, which use 192.168.1.1 as default private IP. But the two most popular NAT devices brands, Linksys and Speedtouch use this and other numbers as their default IP.

source: http://ezinearticles.com/?What-Is-an-IP-Address?&id=5235745

Subnetting is a network design strategy that segregates a larger network into smaller components. While connected through the larger network, each subnetwork or subnet functions with a unique IP address. All systems that are assigned to a particular subnet will share values that are common for both the subnet and for the network as a whole.

A different approach to network construction can be thought of as subnetting in reverse. Known as CIDR, or Classless Inter-Domain Routing, this approach also creates a series of subnetworks. Rather than dividing an existing network into small components, CIDR takes smaller components and connects them into a larger network. This can often be the case when a business is acquired by a larger corporation. Instead of doing away with the network developed and used by the newly acquired business, the corporation chooses to continue operating that network as a subsidiary or an added component of the corporation’s network. In effect, the system of the purchased entity becomes a subnet of the parent company’s network.

Subnetting involves the use of several tools in order to establish the series of subnetworks and allow them to still function as a complete system when necessary. There is the matter of connectivity. In order to allow the subnets to connect, a bridge or routing equipment is normally utilized. The assignment of IP addresses is also important to the proper construction of the subnetworks. Each device or user connected with a given subnetwork will be assigned an IP address utilizing the same prefix. Every subnetwork will have a different IP address prefix.

Splitting network sectors into a series of subnet components has a couple of practical advantages. First, by segregating the larger network into distinct but interconnected subsections, it is often easier to isolate performance issues and repair them without having to shut down the functions taking place in the other subnetworks. The process of subnetting can also enhance the process of maintaining the overall network, making it possible to perform diagnostics or other testing without slowing down or impacting the functionality of other components that make up the larger network.

As Internet usage has continued to become key to the technology many companies use for communication, data storage and transfer, and even general clerical functions, the ability of a traditional Class A, B, or C network to work with optimum efficiency has become more difficult. By employing the process of subnetting, the larger network can add or remove subnets as needed, and assign devices and other resources to a given subsection with relative ease. In terms of logical arrangement, subnetting enhances the ability to manage the larger network as well as help to structure subsections exactly as needed without having to modify protocols for the entire network.

source: http://www.wisegeek.com/what-is-subnetting.htm

This article will explain how to be able to access a PC running Windows 7 from anywhere on the Internet. Before we begin, you should know that most users have a dynamic IP address, one that changes sometimes every 24 hours, which seems not simple to access a computer from the Internet. Here is the solution.

On the Internet, any connected machine (including servers) are identified by a unique IP address. The servers have static IP addresses. Customers, however, often have dynamic IP addresses, which change each time they connect to the Internet. Since IP addresses are not easy to remember, DNS servers are used to associate domain names with corresponding IP addresses.

The service offered by DYNDNS dot ORG is designed for users who have dynamic IP addresses. This service assigns you a free domain name (your choice) and then follows the trail of the changes in your dynamic IP so that the chosen domain name always points to your machine even if the IP is constantly shifting.

The process involves three steps that we are going to review:

- Create an account at DynDNS.COM.

- Configuring your DynDNS account and choose your domain name.

- Configuring the router or computer.

A. Let’s start by opening an account on DYNDNS.COM

1- With the help of your browser, open dyndns dot com.

2- Click on Create Account.

3- Enter your name, email and a password for your account.

4 – Check the box “I agree” and click Create Account.

5- A validation email will be sent.

6- Wait for a few minutes, then open your email client. Locate the Mail from DynDNS.com Support and click on the link it contains. The account is now confirmed.

B. Set up your DynDNS account and select a domain name:

1 – Open your web browser at dyndns.com.

2 – Enter your username in the User field, and password in the Pass field.

3 – Click on Login.

4 – Go to the Services section.

5 – Select Dynamic DNS

6 – Click on Get Started

7 – Enter your name in “Hostname” then choose a domain in the popup menu. For example, choose “myip.net”, which will cause your machine to be identified as “yourname.myip.net”.

8 – Check the box Wildcard

9 – Select Host with IP address option

10 – Click on the link “Use Auto Detected IP address”

11 – Click Create Host

12 – Click on Payment (do not worry it’s free and the service does not require any means of payment)

13 – Click on the Activate Services.

That’s it! You can now pass to the last step.

C. Configuring your PC or router

We must now configure the DNS service to recognize your machine and monitor your IP address. Two cases arise:

CASE 1: You are connected to the Internet via a router, Livebox, Freebox, etc.

In this case, you must configure your router to indicate IP changes to DynDNS. The method varies from one router model to another, but still very similar in all cases.

- Connect to the configuration interface of your router (usually 192.168.1.1)

- In the setup menu (or Advanced Configuration) locate an entry called DDNS, Dynamic DNS, DynDNS or Dynamic DNS.

- Select the DynDNS.org or DynDNS.com service in the proposed list.

- Enter your domain name as defined in step 2 (yourname.myip.net)

- Then enter your DynDNS username and password (as defined in step 1).

- Confirm the changes. The system is now operational.

CASE 2: You are connected to the Internet via a modem

In this case, you must install a small utility on your machine:

- Download the software DynDNS Updater by referring to DynDNS website.

- Run the installation by following the wizard: Click Next, then I Agree, then Next, then Next, then Install and then click Finish.

- The software automatically launches and asks for details of your account

- Enter your DynDNS username and password (as defined in step 1).

- The list of domains associated with your account appears. Check all that must be related to this machine and click Apply. Enjoy!

source: http://ezinearticles.com/?How-to-Access-a-PC-Running-Windows-7-Without-a-Fixed-IP&id=4174940

What are the network setting required for a computer to join a domain?

In order to be able to join a Windows 2000 or Windows Server 2003 domain you must properly configure your XP/W2K computer.

Note: XP Home Edition is not designed to join domains; only workgroups. To join domains, use XP Professional version or above.
Required Settings

A network Interface Card (NIC) – Duh, but unless you have one (or a wireless connection) how do you expect to connect to the server?

Physically be connected to the LAN – Windows XP (and 2000) has an LAN auto sensing feature. Whenever you disconnect from the network, a balloon appears in the task bar area notifying you of the disconnection status. Without a physically connected network the NIC looses it’s IP settings, thus preventing you from connecting to the network (which was disconnected in the first place) or viewing your IP configuration.

A valid IP address – Valid for the network you’re connected to. You can either configure one manually, receive one from a local DHCP Server, or leave it as is and receive an APIPA address. If it’s an APIPA address you’re asking for potential problems, as APIPA and AD do not go together hand-in-hand.

All-time connectivity to the Domain Controller – Or at least one of them. The IP address you’ve configured (or leased) should be good enough to enable you to connect to one of the Domain Controllers on your Domain. Test your connectivity with PING.

A properly configured DNS server – Without a properly configured DNS server your workstation will not be able to connect to the domain. Even if it did (for example you had a working DNS server but you somehow messed it up or shut it down) it will take a lot of time to actually log-on, and many AD related administration tasks will not work.

The DNS server must hold a zone with the exact name of the AD domain you’re trying to join. It also must hold 4 SRV folders (you can tell by the “_” in their name). If it doesn’t, you either misspelled the domain name or DNS zone, or the zone is not configured to accept dynamic registrations, or it’s not a Windows 2000 DNS server, or the Domain Controller does not have a working connection with the DNS server (firewall problems, improper IP configuration, IPSec etc.)

All-time connectivity to the DNS server – Test your connection to the DNS server by PINGing it and performing an NSLOOKUP query.

Local Administrative power – A simple user won’t do. You must be the local Administrator.

Correct domain name, Administrator’s name and password – Misspelled your domain name? You won’t get to the Username and Password prompt!

Got your domain name right? You’ll be asked for a valid username and password. To be safe, enter one that has Domain Admins rights, although you could get away with less, depending on your AD configuration.

No Internet Connection Sharing please – ICS will mess up your network. Do not use it. Use RRAS and NAT instead. It will work if it has to, but ICS and AD do not go together hand-in-hand. You are warned.

source:
http://www.petri.co.il/requirements_when_joining_a_domain.htm

A Windows-based computer that is configured to use DHCP can automatically assign itself an Internet Protocol (IP) address if a DHCP server is not available. For example, this could occur on a network without a DHCP server, or on a network if a DHCP server is temporarily down for maintenance.

With this feature, a Windows computer can assign itself an Internet Protocol (IP) address in the event that a DHCP server is not available or does not exist on the network. This feature makes configuring and supporting a small Local Area Network (LAN) running TCP/IP less difficult.

Note: You may want to read Disable APIPA in Windows 98/ME/2000/XP/2003

The Internet Assigned Numbers Authority (IANA) has reserved 169.254.0.0-169.254.255.255 for Automatic Private IP Addressing. As a result, APIPA provides an address that is guaranteed not to conflict with routable addresses.

After the network adapter has been assigned an IP address, the computer can use TCP/IP to communicate with any other computer that is connected to the same LAN and that is also configured for APIPA or has the IP address manually set to the 169.254.x.y (where x.y is the client’s unique identifier) address range with a subnet mask of 255.255.0.0. Note that the computer cannot communicate with computers on other subnets, or with computers that do not use automatic private IP addressing. Automatic private IP addressing is enabled by default.

Is my computer using APIPA now?

Windows 98/ME
You can also determine whether your computer is using APIPA by using the Winipcfg tool in Windows Millennium Edition, Windows 98, or Windows 98 Second Edition:

Click Start, click Run, type “winipcfg” (without the quotation marks), and then click OK. Click More Info. If the IP Autoconfiguration Address box contains an IP address within the 169.254.x.x range, Automatic Private IP Addressing is enabled. If the IP Address box exists, automatic private IP addressing is not currently enabled.

Windows 2000/XP/2003
For Windows 2000, Windows XP, or Windows Server 2003, you can determine whether your computer is using APIPA by using the IPconfig command at a command prompt:

Click Start, click Run, type “cmd” (without the quotation marks), and then click OK to open a MS-DOS command line window. Type “ipconfig /all” (without the quotation marks), and then hit the ENTER key. If the ‘Autoconfiguration Enabled’ line says “Yes”, and the ‘Autoconfiguration IP Address’ is 169.254.x.y (where x.y is the client’s unique identifier), then the computer is using APIPA. If the ‘Autoconfiguration Enabled’ line says “No”, then the computer is not currently using APIPA.

Examples of Where APIPA May Be Useful

Example 1: No Previous IP Address and no DHCP Server
When your Windows-based computer (configured for DHCP) is initializing, it broadcasts three or more “discover” messages. If a DHCP server does not respond after several discover messages are broadcast, the Windows computer assigns itself a Class B (APIPA) address. Then the Windows computer will display an error message to the user of the computer (providing it has never been assigned an IP address from a DHCP server in the past). The Windows computer will then send out a Discover message every three minutes in an attempt to establish communications with a DHCP server.

Example 2: Previous IP Address and no DHCP Server
The computer checks for the DHCP server and if none are found, an attempt is made to contact the default gateway. If the default gateway replies, then the Windows computer retains the previously-leased IP address. However, if the computer does not receive a response from the default gateway or if none are assigned, then it uses the automatic private IP addressing feature to assign itself an IP address. An error message is presented to the user and discover messages are transmitted every 3 minutes. Once a DHCP server comes on line, a message is generated stating communications have been re-established with a DHCP Server.

Example 3: Lease Expires and no DHCP Server
The Windows-based computer tries to re-establish the lease of the IP address. If the Windows computer does not find a DHCP server, it assigns itself an IP address after generating an error message. The computer then broadcasts a discover message every 3 minutes until a DHCP server comes on line. A message is then generated stating that communications have been re-established with the DHCP Server.

source:
http://www.petri.co.il/what%27s_apipa.htm

Have you ever wondered about the mechanisms that delivered this page to you? Chances are you are sitting at a computer right now, viewing this page in a browser. So, when you clicked on the link for this page, or typed in its URL (uniform resource locator), what happened behind the scenes to bring this page onto your screen?

If you’ve ever been curious about the process, or have ever wanted to know some of the specific mechanisms that allow you to surf the Internet, then read on. In this article, you will learn how Web servers bring pages into your home, school or office.

The Basic Process

Let’s say that you are sitting at your computer, surfing the Web, and you get a call from a friend who says, “I just read a great article! Type in this URL and check it out. It’s at http://www.howstuffworks.com/web-server.htm.” So you type that URL into your browser and press return. And magically, no matter where in the world that URL lives, the page pops up on your screen.

At the most basic level possible, the following diagram shows the steps that brought that page to your screen:

Your browser formed a connection to a Web server, requested a page and received it.

Behind the Scenes

If you want to get into a bit more detail on the process of getting a Web page onto your computer screen, here are the basic steps that occurred behind the scenes:

* The browser broke the URL into three parts:
1. The protocol (“http”)
2. The server name (“www.howstuffworks.com”)
3. The file name (“web-server.htm”)

* The browser communicated with a name server to translate the server name “www.howstuffworks.com” into an IP Address, which it uses to connect to the server machine.

* The browser then formed a connection to the server at that IP address on port 80. (We’ll discuss ports later in this article.)

* Following the HTTP protocol, the browser sent a GET request to the server, asking for the file “http://www.howstuffworks.com/web-server.htm.” (Note that cookies may be sent from browser to server with the GET request — see How Internet Cookies Work for details.)

* The server then sent the HTML text for the Web page to the browser. (Cookies may also be sent from server to browser in the header for the page.)

* The browser read the HTML tags and formatted the page onto your screen.

If you’ve never explored this process before, that’s a lot of new vocabulary. To understand this whole process in detail, you need to learn about IP addresses, ports, protocols.

The Internet

So what is “the Internet”? The Internet is a gigantic collection of millions of computers, all linked together on a computer network. The network allows all of the computers to communicate with one another. A home computer may be linked to the Internet using a phone-line modem, DSL or cable modem that talks to an Internet service provider (ISP). A computer in a business or university will usually have a network interface card (NIC) that directly connects it to a local area network (LAN) inside the business. The business can then connect its LAN to an ISP using a high-speed phone line like a T1 line. A T1 line can handle approximately 1.5 million bits per second, while a normal phone line using a modem can typically handle 30,000 to 50,000 bits per second.

ISPs then connect to larger ISPs, and the largest ISPs maintain fiber-optic “backbones” for an entire nation or region. Backbones around the world are connected through fiber-optic lines, undersea cables or satellite links (see An Atlas of Cyberspaces for some interesting backbone maps). In this way, every computer on the Internet is connected to every other computer on the Internet.

Clients and Servers

In general, all of the machines on the Internet can be categorized as two types: servers and clients. Those machines that provide services (like Web servers or FTP servers) to other machines are servers. And the machines that are used to connect to those services are clients. When you connect to Yahoo! at www.yahoo.com to read a page, Yahoo! is providing a machine (probably a cluster of very large machines), for use on the Internet, to service your request. Yahoo! is providing a server. Your machine, on the other hand, is probably providing no services to anyone else on the Internet. Therefore, it is a user machine, also known as a client. It is possible and common for a machine to be both a server and a client, but for our purposes here you can think of most machines as one or the other.

A server machine may provide one or more services on the Internet. For example, a server machine might have software running on it that allows it to act as a Web server, an e-mail server and an FTP server. Clients that come to a server machine do so with a specific intent, so clients direct their requests to a specific software server running on the overall server machine. For example, if you are running a Web browser on your machine, it will most likely want to talk to the Web server on the server machine. Your Telnet application will want to talk to the Telnet server, your e-mail application will talk to the e-mail server, and so on…

IP Addresses

To keep all of these machines straight, each machine on the Internet is assigned a unique address called an IP address. IP stands for Internet protocol, and these addresses are 32-bit numbers, normally expressed as four “octets” in a “dotted decimal number.” A typical IP address looks like this:

216.27.61.137

The four numbers in an IP address are called octets because they can have values between 0 and 255, which is 28 possibilities per octet.

Every machine on the Internet has a unique IP address. A server has a static IP address that does not change very often. A home machine that is dialing up through a modem often has an IP address that is assigned by the ISP when the machine dials in. That IP address is unique for that session — it may be different the next time the machine dials in. This way, an ISP only needs one IP address for each modem it supports, rather than for each customer.

If you are working on a Windows machine, you can view a lot of the Internet information for your machine, including your current IP address and hostname, with the command WINIPCFG.EXE (IPCONFIG.EXE for Windows 2000/XP). On a UNIX machine, type nslookup at the command prompt, along with a machine name, like www.howstuffworks.com — e.g. “nslookup www.howstuffworks.com” — to display the IP address of the machine, and you can use the command hostname to learn the name of your machine.

As far as the Internet’s machines are concerned, an IP address is all you need to talk to a server. For example, in your browser, you can type the URL http://209.116.69.66 and arrive at the machine that contains the Web server for HowStuffWorks. On some servers, the IP address alone is not sufficient, but on most large servers it is — keep reading for details.

Domain Names

Because most people have trouble remembering the strings of numbers that make up IP addresses, and because IP addresses sometimes need to change, all servers on the Internet also have human-readable names, called domain names. For example, www.howstuffworks.com is a permanent, human-readable name. It is easier for most of us to remember www.howstuffworks.com than it is to remember 209.116.69.66.

The name www.howstuffworks.com actually has three parts:

1. The host name (“www”)
2. The domain name (“howstuffworks”)
3. The top-level domain name (“com”)

Domain names within the “.com” domain are managed by the registrar called VeriSign. VeriSign also manages “.net” domain names. Other registrars (like RegistryPro, NeuLevel and Public Interest Registry) manage the other domains (like .pro, .biz and .org). VeriSign creates the top-level domain names and guarantees that all names within a top-level domain are unique. VeriSign also maintains contact information for each site and runs the “whois” database. The host name is created by the company hosting the domain. “www” is a very common host name, but many places now either omit it or replace it with a different host name that indicates a specific area of the site. For example, in encarta.msn.com, the domain name for Microsoft’s Encarta encyclopedia, “encarta” is designated as the host name instead of “www.”

Name Servers

The whois Command

On a UNIX machine, you can use the whois command to look up information about a domain name. You can do the same thing using the whois form at VeriSign. If you type in a domain name, like “howstuffworks.com,” it will return to you the registration information for that domain, including its IP address.

A set of servers called domain name servers (DNS) maps the human-readable names to the IP addresses. These servers are simple databases that map names to IP addresses, and they are distributed all over the Internet. Most individual companies, ISPs and universities maintain small name servers to map host names to IP addresses. There are also central name servers that use data supplied by VeriSign to map domain names to IP addresses.

If you type the URL “http://www.howstuffworks.com/web-server.htm” into your browser, your browser extracts the name “www.howstuffworks.com,” passes it to a domain name server, and the domain name server returns the correct IP address for www.howstuffworks.com. A number of name servers may be involved to get the right IP address. For example, in the case of www.howstuffworks.com, the name server for the “com” top-level domain will know the IP address for the name server that knows host names, and a separate query to that name server, operated by the HowStuffWorks ISP, may deliver the actual IP address for the HowStuffWorks server machine.

On a UNIX machine, you can access the same service using the nslookup command. Simply type a name like “www.howstuffworks.com” into the command line, and the command will query the name servers and deliver the corresponding IP address to you.

So here it is: The Internet is made up of millions of machines, each with a unique IP address. Many of these machines are server machines, meaning that they provide services to other machines on the Internet. You have heard of many of these servers: e-mail servers, Web servers, FTP servers, Gopher servers and Telnet servers, to name a few. All of these are provided by server machines.

Ports

Any server machine makes its services available to the Internet using numbered ports, one for each service that is available on the server. For example, if a server machine is running a Web server and an FTP server, the Web server would typically be available on port 80, and the FTP server would be available on port 21. Clients connect to a service at a specific IP address and on a specific port.

Each of the most well-known services is available at a well-known port number. Here are some common port numbers:

* echo 7
* daytime 13
* qotd 17 (Quote of the Day)
* ftp 21
* telnet 23
* smtp 25 (Simple Mail Transfer, meaning e-mail)
* time 37
* nameserver 53
* nicname 43 (Who Is)
* gopher 70
* finger 79
* WWW 80

If the server machine accepts connections on a port from the outside world, and if a firewall is not protecting the port, you can connect to the port from anywhere on the Internet and use the service. Note that there is nothing that forces, for example, a Web server to be on port 80. If you were to set up your own machine and load Web server software on it, you could put the Web server on port 918, or any other unused port, if you wanted to. Then, if your machine were known as xxx.yyy.com, someone on the Internet could connect to your server with the URL http://xxx.yyy.com:918. The “:918″ explicitly specifies the port number, and would have to be included for someone to reach your server. When no port is specified, the browser simply assumes that the server is using the well-known port 80.

Protocols

Once a client has connected to a service on a particular port, it accesses the service using a specific protocol. The protocol is the pre-defined way that someone who wants to use a service talks with that service. The “someone” could be a person, but more often it is a computer program like a Web browser. Protocols are often text, and simply describe how the client and server will have their conversation.

Perhaps the simplest protocol is the daytime protocol. If you connect to port 13 on a machine that supports a daytime server, the server will send you its impression of the current date and time and then close the connection. The protocol is, “If you connect to me, I will send you the date and time and then disconnect.” Most UNIX machines support this server. If you would like to try it out, you can connect to one with the Telnet application. In UNIX, the session would look like this:

%telnet web67.ntx.net 13
Trying 216.27.61.137…
Connected to web67.ntx.net.
Escape character is ‘^]’.
Sun Oct 25 08:34:06 1998
Connection closed by foreign host.

On a Windows machine, you can access this server by typing “telnet web67.ntx.net 13″ at the MSDOS prompt.

In this example, web67.ntx.net is the server’s UNIX machine, and 13 is the port number for the daytime service. The Telnet application connects to port 13 (telnet naturally connects to port 23, but you can direct it to connect to any port), then the server sends the date and time and disconnects. Most versions of Telnet allow you to specify a port number, so you can try this using whatever version of Telnet you have available on your machine.

Most protocols are more involved than daytime and are specified in Request for Comment (RFC) documents that are publicly available (see http://sunsite.auc.dk/RFC/ for a nice archive of all RFCs). Every Web server on the Internet conforms to the HTTP protocol, summarized nicely in The Original HTTP as defined in 1991. The most basic form of the protocol understood by an HTTP server involves just one command: GET. If you connect to a server that understands the HTTP protocol and tell it to “GET filename,” the server will respond by sending you the contents of the named file and then disconnecting. Here’s a typical session:

%telnet www.howstuffworks.com 80
Trying 216.27.61.137…
Connected to howstuffworks.com.
Escape character is ‘^]’.
GET http://www.howstuffworks.com/

…
Connection closed by foreign host.

In the original HTTP protocol, all you would have sent was the actual filename, such as “/” or “/web-server.htm.” The protocol was later modified to handle the sending of the complete URL. This has allowed companies that host virtual domains, where many domains live on a single machine, to use one IP address for all of the domains they host. It turns out that hundreds of domains are hosted on 209.116.69.66 — the HowStuffWorks IP address.

Putting It All Together

Now you know a tremendous amount about the Internet. You know that when you type a URL into a browser, the following steps occur:

* The browser breaks the URL into three parts:
1. The protocol (“http”)
2. The server name (“www.howstuffworks.com”)
3. The file name (“web-server.htm”)

* The browser communicates with a name server to translate the server name, “www.howstuffworks.com,” into an IP address, which it uses to connect to that server machine.

* The browser then forms a connection to the Web server at that IP address on port 80.

* Following the HTTP protocol, the browser sends a GET request to the server, asking for the file “http://www.howstuffworks.com/web-server.htm.” (Note that cookies may be sent from browser to server with the GET request — see How Internet Cookies Work for details.)

* The server sends the HTML text for the Web page to the browser. (Cookies may also be sent from server to browser in the header for the page.)

* The browser reads the HTML tags and formats the page onto your screen.

Security

You can see from this description that a Web server can be a pretty simple piece of software. It takes the file name sent in with the GET command, retrieves that file and sends it down the wire to the browser. Even if you take into account all of the code to handle the ports and port connections, you could easily create a C program that implements a simple Web server in less than 500 lines of code. Obviously, a full-blown enterprise-level Web server is more involved, but the basics are very simple.

Most servers add some level of security to the serving process. For example, if you have ever gone to a Web page and had the browser pop up a dialog box asking for your name and password, you have encountered a password-protected page. The server lets the owner of the page maintain a list of names and passwords for those people who are allowed to access the page; the server lets only those people who know the proper password see the page. More advanced servers add further security to allow an encrypted connection between server and browser, so that sensitive information like credit card numbers can be sent on the Internet.

That’s really all there is to a Web server that delivers standard, static pages. Static pages are those that do not change unless the creator edits the page.

Dynamic Pages

But what about the Web pages that are dynamic? For example:

* Any guest book allows you to enter a message in an HTML form, and the next time the guest book is viewed, the page will contain the new entry.

* The whois form at Network Solutions allows you to enter a domain name on a form, and the page returned is different depending on the domain name entered.

* Any search engine lets you enter keywords on an HTML form, and then it dynamically creates a page based on the keywords you enter.

In all of these cases, the Web server is not simply “looking up a file.” It is actually processing information and generating a page based on the specifics of the query. In almost all cases, the Web server is using something called CGI scripts to accomplish this feat.

source: http://computer.howstuffworks.com/web-server.htm

If you are reading this article, you are most likely connected to the Internet and viewing it at the HowStuffWorks Web site. There’s a very good chance that you are using Network Address Translation (NAT) right now.

The Internet has grown larger than anyone ever imagined it could be. Although the exact size is unknown, the current estimate is that there are about 100 million hosts and more than 350 million users actively on the Internet. That is more than the entire population of the United States! In fact, the rate of growth has been such that the Internet is effectively doubling in size each year.

So what does the size of the Internet have to do with NAT? Everything! For a computer to communicate with other computers and Web servers on the Internet, it must have an IP address. An IP address (IP stands for Internet Protocol) is a unique 32-bit number that identifies the location of your computer on a network. Basically, it works like your street address — as a way to find out exactly where you are and deliver information to you.

When IP addressing first came out, everyone thought that there were plenty of addresses to cover any need. Theoretically, you could have 4,294,967,296 unique addresses (232). The actual number of available addresses is smaller (somewhere between 3.2 and 3.3 billion) because of the way that the addresses are separated into classes, and because some addresses are set aside for multicasting, testing or other special uses.

With the explosion of the Internet and the increase in home networks and business networks, the number of available IP addresses is simply not enough. The obvious solution is to redesign the address format to allow for more possible addresses. This is being developed (called IPv6), but will take several years to implement because it requires modification of the entire infrastructure of the Internet.

This is where NAT (RFC 1631) comes to the rescue. Network Address Translation allows a single device, such as a router, to act as an agent between the Internet (or “public network”) and a local (or “private”) network. This means that only a single, unique IP address is required to represent an entire group of computers.

What Does NAT Do?

NAT is like the receptionist in a large office. Let’s say you have left instructions with the receptionist not to forward any calls to you unless you request it. Later on, you call a potential client and leave a message for that client to call you back. You tell the receptionist that you are expecting a call from this client and to put her through.

The client calls the main number to your office, which is the only number the client knows. When the client tells the receptionist that she is looking for you, the receptionist checks a lookup table that matches your name with your extension. The receptionist knows that you requested this call, and therefore forwards the caller to your extension.

Developed by Cisco, Network Address Translation is used by a device (firewall, router or computer) that sits between an internal network and the rest of the world. NAT has many forms and can work in several ways:

* Static NAT – Mapping an unregistered IP address to a registered IP address on a one-to-one basis. Particularly useful when a device needs to be accessible from outside the network.

In static NAT, the computer with the IP address of 192.168.32.10 will always translate to 213.18.123.110.

* Dynamic NAT – Maps an unregistered IP address to a registered IP address from a group of registered IP addresses.

In dynamic NAT, the computer with the IP address 192.168.32.10 will translate to the first available address in the range from 213.18.123.100 to 213.18.123.150.

* Overloading – A form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address by using different ports. This is known also as PAT (Port Address Translation), single address NAT or port-level multiplexed NAT.

In overloading, each computer on the private network is translated to the same IP address (213.18.123.100), but with a different port number assignment.

* Overlapping – When the IP addresses used on your internal network are registered IP addresses in use on another network, the router must maintain a lookup table of these addresses so that it can intercept them and replace them with registered unique IP addresses. It is important to note that the NAT router must translate the “internal” addresses to registered unique addresses as well as translate the “external” registered addresses to addresses that are unique to the private network. This can be done either through static NAT or by using DNS and implementing dynamic NAT.

The internal IP range (237.16.32.xx) is also a registered range used by another network. Therefore, the router is translating the addresses to avoid a potential conflict with another network. It will also translate the registered global IP addresses back to the unregistered local IP addresses when information is sent to the internal network.

The internal network is usually a LAN (Local Area Network), commonly referred to as the stub domain. A stub domain is a LAN that uses IP addresses internally. Most of the network traffic in a stub domain is local, so it doesn’t travel outside the internal network. A stub domain can include both registered and unregistered IP addresses. Of course, any computers that use unregistered IP addresses must use Network Address Translation to communicate with the rest of the world.

NAT Configuration

NAT can be configured in various ways. In the example below, the NAT router is configured to translate unregistered (inside, local) IP addresses, that reside on the private (inside) network, to registered IP addresses. This happens whenever a device on the inside with an unregistered address needs to communicate with the public (outside) network.

* An ISP assigns a range of IP addresses to your company. The assigned block of addresses are registered, unique IP addresses and are called inside global addresses. Unregistered, private IP addresses are split into two groups. One is a small group (outside local addresses) that will be used by the NAT routers. The other, much larger group, known as inside local addresses, will be used on the stub domain. The outside local addresses are used to translate the unique IP addresses, known as outside global addresses, of devices on the public network.

* Most computers on the stub domain communicate with each other using the inside local addresses.
* Some computers on the stub domain communicate a lot outside the network. These computers have inside global addresses, which means that they do not require translation.
* When a computer on the stub domain that has an inside local address wants to communicate outside the network, the packet goes to one of the NAT routers.
* The NAT router checks the routing table to see if it has an entry for the destination address. If it does, the NAT router then translates the packet and creates an entry for it in the address translation table. If the destination address is not in the routing table, the packet is dropped.
* Using an inside global address, the router sends the packet on to its destination.
* A computer on the public network sends a packet to the private network. The source address on the packet is an outside global address. The destination address is an inside global address.
* The NAT router looks at the address translation table and determines that the destination address is in there, mapped to a computer on the stub domain.
* The NAT router translates the inside global address of the packet to the inside local address, and sends it to the destination computer.

NAT overloading utilizes a feature of the TCP/IP protocol stack, multiplexing, that allows a computer to maintain several concurrent connections with a remote computer (or computers) using different TCP or UDP ports. An IP packet has a header that contains the following information:

* Source Address – The IP address of the originating computer, such as 201.3.83.132
* Source Port – The TCP or UDP port number assigned by the originating computer for this packet, such as Port 1080
* Destination Address – The IP address of the receiving computer, such as 145.51.18.223
* Destination Port – The TCP or UDP port number that the originating computer is asking the receiving computer to open, such as Port 3021

The addresses specify the two machines at each end, while the port numbers ensure that the connection between the two computers has a unique identifier. The combination of these four numbers defines a single TCP/IP connection. Each port number uses 16 bits, which means that there are a possible 65,536 (216) values. Realistically, since different manufacturers map the ports in slightly different ways, you can expect to have about 4,000 ports available.

Dynamic NAT and Overloading

Here’s how dynamic NAT works:

* An internal network (stub domain) has been set up with IP addresses that were not specifically allocated to that company by IANA (Internet Assigned Numbers Authority), the global authority that hands out IP addresses. These addresses should be considered non-routable since they are not unique.

* The company sets up a NAT-enabled router. The router has a range of unique IP addresses given to the company by IANA.

* A computer on the stub domain attempts to connect to a computer outside the network, such as a Web server.

* The router receives the packet from the computer on the stub domain.

* The router saves the computer’s non-routable IP address to an address translation table. The router replaces the sending computer’s non-routable IP address with the first available IP address out of the range of unique IP addresses. The translation table now has a mapping of the computer’s non-routable IP address matched with the one of the unique IP addresses.

* When a packet comes back from the destination computer, the router checks the destination address on the packet. It then looks in the address translation table to see which computer on the stub domain the packet belongs to. It changes the destination address to the one saved in the address translation table and sends it to that computer. If it doesn’t find a match in the table, it drops the packet.

* The computer receives the packet from the router. The process repeats as long as the computer is communicating with the external system.

Here’s how overloading works:

* An internal network (stub domain) has been set up with non-routable IP addresses that were not specifically allocated to that company by IANA.

* The company sets up a NAT-enabled router. The router has a unique IP address given to the company by IANA.

* A computer on the stub domain attempts to connect to a computer outside the network, such as a Web server.

* The router receives the packet from the computer on the stub domain.

* The router saves the computer’s non-routable IP address and port number to an address translation table. The router replaces the sending computer’s non-routable IP address with the router’s IP address. The router replaces the sending computer’s source port with the port number that matches where the router saved the sending computer’s address information in the address translation table. The translation table now has a mapping of the computer’s non-routable IP address and port number along with the router’s IP address.

* When a packet comes back from the destination computer, the router checks the destination port on the packet. It then looks in the address translation table to see which computer on the stub domain the packet belongs to. It changes the destination address and destination port to the ones saved in the address translation table and sends it to that computer.

* The computer receives the packet from the router. The process repeats as long as the computer is communicating with the external system.

* Since the NAT router now has the computer’s source address and source port saved to the address translation table, it will continue to use that same port number for the duration of the connection. A timer is reset each time the router accesses an entry in the table. If the entry is not accessed again before the timer expires, the entry is removed from the table.

Stub Domains

The NAT router stores the IP address and port number of each computer in the address translation table. It then replaces the IP address with its own registered IP address and the port number corresponding to the location, in the table, of the entry for that packet’s source computer. So any external network sees the NAT router’s IP address and the port number assigned by the router as the source-computer information on each packet.

You can still have some computers on the stub domain that use dedicated IP addresses. You can create an access list of IP addresses that tells the router which computers on the network require NAT. All other IP addresses will pass through untranslated.

The number of simultaneous translations that a router will support are determined mainly by the amount of DRAM (Dynamic Random Access Memory) it has. But since a typical entry in the address-translation table only takes about 160 bytes, a router with 4 MB of DRAM could theoretically process 26,214 simultaneous translations, which is more than enough for most applications.

IANA has set aside specific ranges of IP addresses for use as non-routable, internal network addresses. These addresses are considered unregistered (for more information check out RFC 1918: Address Allocation for Private Internets, which defines these address ranges). No company or agency can claim ownership of unregistered addresses or use them on public computers. Routers are designed to discard (instead of forward) unregistered addresses. What this means is that a packet from a computer with an unregistered address could reach a registered destination computer, but the reply would be discarded by the first router it came to.

There is a range for each of the three classes of IP addresses used for networking:

* Range 1: Class A – 10.0.0.0 through 10.255.255.255
* Range 2: Class B – 172.16.0.0 through 172.31.255.255
* Range 3: Class C – 192.168.0.0 through 192.168.255.255

Although each range is in a different class, your are not required to use any particular range for your internal network. It is a good practice, though, because it greatly diminishes the chance of an IP address conflict.

Security and Administration

Implementing dynamic NAT automatically creates a firewall between your internal network and outside networks, or between your internal network and the Internet. NAT only allows connections that originate inside the stub domain. Essentially, this means that a computer on an external network cannot connect to your computer unless your computer has initiated the contact. You can browse the Internet and connect to a site, and even download a file; but somebody else cannot latch onto your IP address and use it to connect to a port on your computer.

In specific circumstances, Static NAT, also called inbound mapping, allows external devices to initiate connections to computers on the stub domain. For instance, if you wish to go from an inside global address to a specific inside local address that is assigned to your Web server, Static NAT would enable the connection.

Static NAT (inbound mapping) allows a computer on the stub domain to maintain a specific address when communicating with devices outside the network.

Some NAT routers provide for extensive filtering and traffic logging. Filtering allows your company to control what type of sites employees visit on the Web, preventing them from viewing questionable material. You can use traffic logging to create a log file of what sites are visited and generate various reports from it.

NAT is sometimes confused with proxy servers, but there are definite differences between them. NAT is transparent to the source and to destination computers. Neither one realizes that it is dealing with a third device. But a proxy server is not transparent. The source computer knows that it is making a request to the proxy server and must be configured to do so. The destination computer thinks that the proxy server IS the source computer, and deals with it directly. Also, proxy servers usually work at layer 4 (transport) of the OSI Reference Model or higher, while NAT is a layer 3 (network) protocol. Working at a higher layer makes proxy servers slower than NAT devices in most cases.

NAT operates at the Network layer (layer 3) of the OSI Reference Model — this is the layer that routers work at.

A real benefit of NAT is apparent in network administration. For example, you can move your Web server or FTP server to another host computer without having to worry about broken links. Simply change the inbound mapping at the router to reflect the new host. You can also make changes to your internal network easily, because the only external IP address either belongs to the router or comes from a pool of global addresses.

NAT and DHCP (dynamic host configuration protocol ) are a natural fit. You can choose a range of unregistered IP addresses for your stub domain and have the DHCP server dole them out as necessary. It also makes it much easier to scale up your network as your needs grow. You don’t have to request more IP addresses from IANA. Instead, you can just increase the range of available IP addresses configured in DHCP to immediately have room for additional computers on your network.

Multi-homing

As businesses rely more and more on the Internet, having multiple points of connection to the Internet is fast becoming an integral part of their network strategy. Multiple connections, known as multi-homing, reduces the chance of a potentially catastrophic shutdown if one of the connections should fail.

In addition to maintaining a reliable connection, multi-homing allows a company to perform load-balancing by lowering the number of computers connecting to the Internet through any single connection. Distributing the load through multiple connections optimizes the performance and can significantly decrease wait times.

Multi-homed networks are often connected to several different ISPs (Internet Service Providers). Each ISP assigns an IP address (or range of IP addresses) to the company. Routers use BGP (Border Gateway Protocol), a part of the TCP/IP protocol suite, to route between networks using different protocols. In a multi-homed network, the router utilizes IBGP (Internal Border Gateway Protocol) on the stub domain side, and EBGP (External Border Gateway Protocol) to communicate with other routers.

Multi-homing really makes a difference if one of the connections to an ISP fails. As soon as the router assigned to connect to that ISP determines that the connection is down, it will reroute all data through one of the other routers.

NAT can be used to facilitate scalable routing for multi-homed, multi-provider connectivity.

source: http://www.howstuffworks.com/nat.htm