Network Address Translation (NAT) is 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. Developed by Cisco, the NAT process relies on a device (usually a router) to make all necessary IP address translations where the LAN meets the Internet.
NAT serves three practical purposes:
- To provide a type of firewall by hiding internal IP addresses
- To enable a company to use more internal IP addresses*
- To allow a company to merge multiple ISDN connections to form a single Internet connection
*Since the IP addresses are only used internally, there is no risk of external conflict with those used by other companies or organizations see overlapping overview below.
Main types of NAT
- Dynamic NAT refers to the process of translating private IP addresses to public IP addresses within a specified range (called a NAT pool).
- Static NAT refers to the process of translating private IP addresses one-to-one to the same public IP address.
- Sometimes called Port Address Translation (PAT) or single address NAT, overloading refers to a version of dynamic NAT in which all of the private IP addresses are translated to the same public IP address using different ports.
- Overlapping occurs when the private IP addresses in an internal network (sometimes called a stub domain) are the same as those in an external network, meaning the NAT router must keep a lookup table so it can compare the two and translate accordingly.