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In computer networking, network address translation (NAT) is the process of modifying network address information in datagram packet headers while in transit across a traffic routing device for the purpose of remapping a given address space into another.
Most often today, NAT is used in conjunction with network masquerading (or IP masquerading) which is a technique that hides an entire address space, usually consisting of private network addresses (RFC 1918), behind a single IP address in another, often public address space. This mechanism is implemented in a routing device that uses stateful translation tables to map the "hidden" addresses into a single address and then rewrites the outgoing Internet Protocol (IP) packets on exit so that they appear to originate from the router. In the reverse communications path, responses are mapped back to the originating IP address using the rules ("state") stored in the translation tables. The translation table rules established in this fashion are flushed after a short period without new traffic refreshing their state.
As described, the method only allows transit traffic through the router when it is originating in the masqueraded network, since this establishes the translation tables. However, most NAT devices today allow the network administrator to configure translation tables entries for permanent use. This feature is often referred to as "static NAT" or port forwarding and allows traffic originating in the 'outside' network to reach designated hosts in the masqueraded network.
Because of the popularity of this technique, see below, the term NAT has become virtually synonymous with the method of IP masquerading.
Network address translation has serious consequences (see below, Drawbacks, Benefits) on the quality of Internet connectivity and requires careful attention to the details of its implementation. As a result, many methods have been devised to alleviate the issues encountered. See article on NAT traversal.
Overview
In the mid-1990's NAT became a popular tool for alleviating the IPv4 address shortage. NAT has proven particularly popular in countries that (for historical reasons) had fewer address space blocks allocated per capita.[citation needed] than, for example, the United States. It has become a standard, 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 (see gateway). However, NAT breaks the originally envisioned model of IP end-to-end connectivity across the Internet and introduces complications in communication between hosts and has performance impacts.
NAT obscures an internal network's structure: all traffic appears to outside parties as if it originates from the gateway machine.
It has been argued that the wide adoption of IPv6 would make NAT unnecessary, as NAT is a method of handling the shortage of the IPv4 address space.[citation needed]
Network address translation involves re-writing the source and/or destination IP addresses and usually also the TCP/UDP port numbers of IP packets as they pass through the NAT. Checksums (both IP and TCP/UDP) must also be rewritten to take account of the changes.
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Most often today, NAT is used in conjunction with network masquerading (or IP masquerading) which is a technique that hides an entire address space, usually consisting of private network addresses (RFC 1918), behind a single IP address in another, often public address space. This mechanism is implemented in a routing device that uses stateful translation tables to map the "hidden" addresses into a single address and then rewrites the outgoing Internet Protocol (IP) packets on exit so that they appear to originate from the router. In the reverse communications path, responses are mapped back to the originating IP address using the rules ("state") stored in the translation tables. The translation table rules established in this fashion are flushed after a short period without new traffic refreshing their state.
As described, the method only allows transit traffic through the router when it is originating in the masqueraded network, since this establishes the translation tables. However, most NAT devices today allow the network administrator to configure translation tables entries for permanent use. This feature is often referred to as "static NAT" or port forwarding and allows traffic originating in the 'outside' network to reach designated hosts in the masqueraded network.
Because of the popularity of this technique, see below, the term NAT has become virtually synonymous with the method of IP masquerading.
Network address translation has serious consequences (see below, Drawbacks, Benefits) on the quality of Internet connectivity and requires careful attention to the details of its implementation. As a result, many methods have been devised to alleviate the issues encountered. See article on NAT traversal.
Overview
In the mid-1990's NAT became a popular tool for alleviating the IPv4 address shortage. NAT has proven particularly popular in countries that (for historical reasons) had fewer address space blocks allocated per capita.[citation needed] than, for example, the United States. It has become a standard, 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 (see gateway). However, NAT breaks the originally envisioned model of IP end-to-end connectivity across the Internet and introduces complications in communication between hosts and has performance impacts.
NAT obscures an internal network's structure: all traffic appears to outside parties as if it originates from the gateway machine.
It has been argued that the wide adoption of IPv6 would make NAT unnecessary, as NAT is a method of handling the shortage of the IPv4 address space.[citation needed]
Network address translation involves re-writing the source and/or destination IP addresses and usually also the TCP/UDP port numbers of IP packets as they pass through the NAT. Checksums (both IP and TCP/UDP) must also be rewritten to take account of the changes.
Read more ...
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