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

A network router is a small electronic device that allows you build a home network simply. The home router serves as the core or “centerpiece” of the network to which computers, printers and other devices can be connected. Networking with a router helps you to (for example):

* share files between computers
* share an Internet connection between computers
* share a printer
* connect your game console or other home entertainment equipment to the Internet

Routers are not necessarily required to build a network. For example, you can connect two computers directly to each other with just a cable (or without wires in some cases). Home routers offer convenience and easier maintenance as your network grows.

Choosing a Network Router
You can choose from among several different types of home network router products. The two most common types in popular usage are the 802.11b and 802.11g WiFi models. 802.11g is the newer technology, but 802.11b routers often can do the job for an even lower cost.

Installing a Network Router
Network routers receive their power from an ordinary home electrical socket. When powered on, lights (LEDs) signify the unit is operating.

Network routers must be carefully configured when they are first installed. Like computers and other devices on the home network, routers must be set up with IP addresses. Routers also offer optional (but strongly recommended) security features.

Routers contain built-in software to enable setup. You access this software through your Web browser on any computer connected to the router.

Connecting Computers to a Router
The most basic use of a network router involves file sharing (copying files) between multiple computers. You do not technically need a router to set up file sharing (or a home network), but using a router greatly simplifies the task, especially when three or more computers are involved.

Home routers provide connection points (called “ports” or “jacks”) for you to connect computers with Ethernet cables. Plug one end of the cable into the router and the other into the computer’s Ethernet network adapter. Wireless routers alternatively allow computers to connect via WiFi technology, if the computer possesses a WiFi network adapter.

Connecting an Internet Modem to the Router

The ability of a network router to share your Internet connection throughout the residence is a key selling point of these boxes. Internet connection sharing can be set up without a router using alternative methods, but once again, having a router greatly simplifies the task.

To use your router for Internet sharing, connect your Internet modem to the appropropriate router jack designed for this purpose. Many network routers allow broadband modems to be connected with either a USB cable or an Ethernet cable. A few network routers even allow traditional dialup modems to be connected via serial cables to a built-in serial port.
Connecting a Printer to the Router
Sharing one printer between multiple home computers is often desired but surprisingly difficult to achieve. Without a router, people connect their printer to one computer designated as the printer host. This host computer must be specially configured, and it must also be operating whenever anyone needs to use the printer. Moving this responsibility from a host computer to a router makes both network setup and using the printer easier.

Normally you can connect your printer to the router using a USB cable or a USB-to-Ethernet cable. Alternatively, wireless print server hardware also exists. A print server connects to your printer’s USB jack and in turn makes a WiFi connection to a wireless router. A few routers contain built-in print server capabilitity, providing a built-in parallel port for cabling a printer directly.
Connecting Home Entertainment Equipment to the Router
You can connect games consoles such as Xbox, set-top devices like TiVo, and other home entertainment equipment to network routers. Networking home entertaining equipment with a router allows these devices to reach the Internet when you have Internet sharing in place there.

Wireless game adapters (also known as wireless bridges) make WiFi connections and USB-to-Ethernet cables make cabled connections to the router for this type of equipment.
Other Uses of a Network Router
A few other types of devices can be added to a network router for special-purpose applications. Video surveillance cameras, for example, can be connected to a router to allow real-time viewing of video feeds from any computer on the home network (or even remotely over the Internet). VoIP analog terminal adapters (ATAs) will often be connected to routers for enabling Internet VoIP call services.

In WiFi networks, routers can be joined with other devices (called range extenders or signal boosters) that increase the overall reach (range) of the wireless signal. Some people do this to share their home network with a neighbor. Wireless routers can sometimes be connected to each other for a similar purpose, but care must be taken to avoid conflicts or interference between the two devices.

source: http://compnetworking.about.com/od/homenetworking/a/routernetworks.htm

This guide explains how to set up a router for home computer networks. The exact names of configuration settings on a network router vary depending on the model and whether it is wired or wireless. However, this general procedure will guide you through the process for the common kinds of home network equipment.

1. Choose a convenient location to begin installing your router such as an open floor space or table. This does not need to be the permanent location of the device. Particularly for wireless routers, you may find it necessary to re-position the unit after installing it as the cables / signals may not reach all areas needed. At the beginning, its better to choose a location where it’s easiest to work with the router and worry about final placement later.

2. Plug in the router’s electrical power source, then turn on the router by pushing the power button.

3. (Optional) Connect your Internet modem to the router. Most network modems connect via an Ethernet cable but USB connections are becoming increasingly common. The cable plugs into the router jack named “WAN” or “uplink” or “Internet.” After connecting the cable, be sure to power cycle (turn off and turn back on) the modem to ensure the router recognizes it.

4. Connect one computer to the router. Even if the router is a wireless model, connect this first computer to the router via a network cable. Using a cable during router installation ensures the maximum reliability of the equipment. Once a wireless router installation is complete, the computer can be changed over to a wireless connection if desired.

5. Open the router’s administration tool. From the computer connected to the router, first open your Web browser. Then enter the router’s address for network administration in the Web address field and hit return to reach the router’s home page.

Many routers are reached by either the Web address “http://192.168.1.1″ or “http://192.168.0.1″ Consult your router’s documentation to determine the exact address for your model. Note that you do not need a working Internet connection for this step.

6. Log in to the router. The router’s home page will ask you for a username and password. Both are provided in the router’s documentation. You should change the router’s password for security reasons, but do this after the installation is complete to avoid unnecessary complications during the basic setup.

7. If you want your router to connect to the Internet, you must enter Internet connection information into that section of the router’s configuration (exact location varies). If using DSL Internet, you may need to enter the PPPoE username and password. Likewise, if you have been issued a static IP address by your provider (you would need to have requested it), the static IP fields (including network mask and gateway) given to you by the provider must also must be set in the router.

8. If you were using a primary computer or an older network router to connect to the Internet, your provider may require you to update the MAC address of the router with the MAC address of the device you were using previously. Read How to Change a MAC Address for a detailed description of this process.

9. If this is a wireless router, change the network name (often called SSID). While the router comes to you with a network name set at the factory, you will never want to use this name on your network. Read How to Change the Router SSID for detailed instructions.

10. Verify the network connection is working between your one computer and the router. To do this, you must confirmed that the computer has received IP address information from the router. See How to Find IP Addresses for a description of this process.

11. (If applicable) Verify your one computer can connect to the Internet properly. Open your Web browser and visit a few Internet sites such as http://compnetworking.about.com/.

12. Connect additional computers to the router as needed. If connecting wirelessly, ensure the network name (SSID) of each is computer matches that of the router.

13. Finally, configure additional network security features as desired to guard your systems against Internet attackers. These WiFi Home Network Security Tips offer a good checklist to follow.

Tips:

When connecting devices with network cables, be sure each end of the cable connects tightly. Loose cables are one of the most common sources of network setup problems.

What You Need:

* A network router (wireless or wired)
* Network adapters installed on all devices to be connected to the router
* A working Internet modem (optional)
* A Web browser installed at least one computer in the network

source: http://compnetworking.about.com/od/homenetworking/ht/routerconfigure.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