NAT

NAT is an Acronym for Network Address Translation, an Internet standard that enables a local-area network (LAN) to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. A NAT box located where the LAN meets the Internet makes all necessary IP address translations.
NAT serves three main purposes:
* Provides a type of firewall by hiding internal IP addresses
* Enables a company to use more internal IP addresses. Since they're used internally only, there's no possibility of conflict with IP addresses used by other companies and organizations.
* Allows a company to combine multiple ISDN connections into a single Internet connection.
Also see Dynamic NAT and Static NAT.