A NAT (Network Address Translation or Network Address Translator) is the virtualization of Internet Protocol (IP) addresses. NAT helps improve security and decrease the number of IP addresses an organization needs.
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NAT gateways sit between two networks, the inside network and the outside network. Systems on the inside network are typically assigned IP addresses that cannot be routed to external networks (e.g., networks in the 10.0.0.0/8 block). A few externally valid IP addresses are assigned to the gateway. The gateway makes outbound traffic from an inside system appear to be coming from one of the valid external addresses. It takes incoming traffic aimed at a valid external address and sends it to the correct internal system. This helps ensure security, since each outgoing or incoming request must go through a translation process that also offers the opportunity to qualify or authenticate incoming streams and match them to outgoing requests, for example.
NAT conserves the number of globally valid IP addresses a company needs, and in combination with Classless Inter-Domain Routing (CIDR) has done a lot to extend the useful life of IPv4 as a result. NAT is described in general terms in IETF RFC 1631.
- From a local IP address to one global IP address statically;
- From a local IP address to any of a rotating pool of global IP addresses a company may have;
- From a local IP address plus a particular TCP port to a global IP address or one in a pool of ports;
- From a global IP address to any of a pool of local IP addresses on a round-robin basis.
In some cases, network administrators don't define simple mappings. Instead they define policies that allow the gateway device to assign mappings based on the intended destination ("pick this external address for communications to partner A's network; pick that external address for communications to partner B's"), or on the protocols being used ("assign out of this pool for HTTP traffic, that pool for HTTPS") or on other factors.
A newer role for NAT focuses on translating IPv4 addresses to IPv6, and vice versa, to provide integration of IPv4 infrastructure and end-nodes into IPv6 environments, and allow IPv6 services to interact with IPv4 systems.