Network / Port Address Translation

Network / Port Address Translation(NAT) is the process of modifying IP address information in IP packet headers while it is in transit across a traffic routing device. The simplest type of NAT provides a one to one translation of IP addresses. In this type of NAT, only the IP addresses, IP header checksum and any higher level checksums that include the IP address need to be changed. The rest of the packet can be left untouched. Basic NATs can be used when there is a requirement to interconnect two IP networks with incompatible addressing.

In the mid-1990s NAT became a popular tool for alleviating the consequences of IPv4 address exhaustion. It has become a common, indispensable feature in routers for home and small-office Internet connections. Most systems using NAT do so in order to enable multiple hosts on a private network to access the Internet using a single public IP address.

However, NAT  has serious drawbacks on the quality of Internet connectivity and requires careful attention to the details of its implementation. In particular, all types of NAT break the originally envisioned model of IP end-to-end connectivity across the Internet and Network Address Port Translation(NAPT) makes it difficult for systems behind a NAT to accept incoming communications. As a result, NAT traversal methods have been devised to alleviate the issues encountered.

References
http://en.wikipedia.org/wiki/Network_address_translation