Network/Port Address Translation

What is NAT?

The Network Address Translation (NAT) 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. The shortage of IP addresses is only one reason to use NAT, it also helps to improve security by reusing IP addresses. The NAT router translates traffic coming into and leaving the private network. 

The diagram below shows the overview of the NAT routing traffic between private(local) and global(Internet) network.

The different types of NAT.

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  IP addresses to a single  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.


References:


http://computer.howstuffworks.com/nat.htm
http://computer.howstuffworks.com/nat1.htm