Network and Address Terms

The NAT implementation defines an address realm as either inside or outside, with the router that is running NAT acting as the defining boundary between the two realms.

From a NAT perspective, an inside network is the local portion of a network that uses private, not publicly routable IP addresses that you want to translate. An outside network is the public portion of a network that uses legitimate, publicly routable IP addresses to which you want private hosts to connect.

The addresses that are translated by NAT between address realms are labeled as inside or outside, and as local or global. When reading the terms in the following sections, keep the following definitions in mind:

Inside Local Addresses

The inside local address is a configured IP address that is assigned to a host on the inside network. Addresses may be globally unique (not requiring translation), allocated from the private address space defined in RFC 1918, or officially allocated to some other organization.

Inside Global Addresses

The inside global address is the translated IP address of an inside host as seen by an outside host and network. Addresses may be allocated from a globally unique address space (often provided by the ISP, if the inside address is connected to the global Internet).

Outside Local Addresses

The outside local address is the translated IP address of an outside host as it appears to the inside network. Addresses may be globally unique (not requiring translation), allocated from the private address space defined in RFC 1918, or officially allocated to some other organization.

Outside Global Addresses

The outside global address is the configured, publicly routable IP address assigned to a host on the outside network.