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Network Address Translation (NAT) is a routing protocol that allows 
your network to become a private network that is isolated from, yet 
connected to the Internet. It does this by changing the IP address of 
packets from a global IP address usable on the Internet to a local IP 
address usable on your private network (but not on the Internet) and 
vice-versa. 

NAT has two major benefits. First, NAT allows many users to access 
the Internet using a small number or even a single global IP address. 
This can greatly reduce the costs associated with Internet access and 
also helps alleviate the current shortage of Internet IP addresses. 
Secondly, the NAT process provides some security found in a firewall, 
hiding your local network from Internet users, providing a degree of 
security to your Internet connection. 

To be successfully implemented, NAT should be used only when the 
majority of network traffic remains on the local network. In cases 
where a large percentage of network traffic is destined for the Internet, 
NAT can adversely affect the speed and performance of your Internet 
connection. Also, your network servers such as ftp servers, web 
servers or mail servers will probably need to be assigned static NAT 
IP addresses so their IP addresses remain consistent. This issue will be 
further discussed later. 

Network Address Port Translation (NAPT) is a subset of NAT where 
many local IP addresses and their TCP/UDP port numbers are 
translated to a single global IP address and it’s TCP/UDP port number. 
In this document, the term NAT will refer to both NAT and NAPT 
unless otherwise stated.