What does the NAT table do?

Answer 1

NAT - Network Address Translation

NAT is one of the basic fundamental functions of a router. If you want to share your internet connection with more than one device, you have to use a router (most Internet Service Providers will supply one to you as part of their package). The router acts as your gateway to Internet, taking complete control of your public IP address (the one assigned to you by your ISP) and assigns private IP addresses to all the devices connected to the router (your computers, your mobile phones, your network printers, etc). Those devices are essentially invisible to the Internet; only the router is exposed.

NAT is essential whenever two separate networks (such as your private home network and the public Internet) require access to each other but their IP address ranges are incompatible with each other. This is the case with all private networks as they all share a common range of IP addresses. In order to route calls from one network to the other, your router uses two IP addresses -- one outward-facing (public) and one inward-facing (private). Your router than acts as the network's gateway to the Internet and effectively hides your private network from the Internet.

When a device sends a request for information from another device on the network, the source device will send a datagram containing the request, along with its own private return address and the address of the device that has the information (all wrapped up in a data packet), which is then forwarded to the router. If the device that has the information is on the same network (such as any device on your home network), then the packet is routed directly to that device without using NAT. However if the device is on another network (such as the Internet), then the router uses NAT to replace the private IP address in the packet with the router's public IP address, and forwards the packet to your service provider. When the requested information comes back from the service provider (usually as a series of data packets), NAT replaces the public IP address in each returned packet with the private IP address of the original device, and forwards the packets to that device.

All of this is achieved by maintaining a NAT table, so the router knows where each packet returned from the service provider is intended to be sent, based upon the data packets it initially received and stored in the table. When multiple computers make similar requests at the same time (as is possible), the router must provide some means of differentiating between those requests. Often this will mean altering the port numbers or adding an anonymous token into the packet to ensure every packet is unambiguously identified.

Example:

You want to look at the wiki answers webpage. Your computer sends a data packet to the router containing its own private IP address, the address of the webserver (which it gets from the Domain Name Server provided by your ISP) and the datagram that describes the request for information. The router looks at the data packet and sees that your computer is requesting information from another network (the Internet), changes the IP address from the private IP address to the public IP address, updates the NAT table accordingly and then forwards the packet to the service provider who will then route it to the webserver. Now the webserver knows where to send the requested information to (your router, via your service provider). When your router receives the requested information from your service provider, it uses the information it stored in the NAT table to restore the original private IP address and then forwards the packet to your computer. There may be more than one packet returned to your computer but the end result is the webpage you requested is displayed in your browser, just as if wiki answers were part of your own private network.