Network address translation (NAT) is a method of mapping an IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device. The technique was originally used to bypass the need to assign a new address to every host when a network was moved, or when the upstream Internet service provider was replaced, but could not route the network's address space. It has become a popular and essential tool in conserving global address space in the face of IPv4 address exhaustion. One Internet-routable IP address of a NAT gateway can be used for an entire private network.
As network address translation modifies the IP address information in packets, NAT implementations may vary in their specific behavior in various addressing cases and their effect on network traffic. The specifics of NAT behavior are not commonly documented by vendors of equipment containing NAT implementations.
The simplest type of NAT provides a one-to-one translation of IP addresses (RFC 1631). RFC 2663 refers to this type of NAT as basic NAT; it is also called a one-to-one NAT. In this type of NAT, only the IP addresses, IP header checksum, and any higher-level checksums that include the IP address are changed. Basic NAT can be used to interconnect two IP networks that have incompatible addressing.
The majority of network address translators map multiple private hosts to one publicly exposed IP address.
Here is a typical configuration:
A local network uses one of the designated private IP address subnets (RFC 1918).
The network has a router having both a private and a public address. The private address is used by the router for communicating with other devices in the private local network. The public address (typically assigned by an Internet service provider) is used by the router for communicating with the rest of the Internet.
As traffic passes from the network to the Internet, the router translates the source address in each packet from a private address to the router's own public address.
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