NAT and PAT Operation

NAT can be used to perform several functions, including the following:

■ Static address translation: Establishes a one-to-one mapping between inside local and global addresses.

■ Dynamic source address translation: Establishes a dynamic mapping between the inside local and global addresses by associating the local addresses to be translated with a pool of addresses from which to allocate global addresses. The router creates translations as needed.

■ Address overloading: Can conserve addresses in the inside global address pool by allowing source ports in TCP connections or User Datagram Protocol (UDP) conversations to be translated. When different inside local addresses map to the same inside global address, TCP or UDP port numbers are used to distinguish between them. Address overloading is also known as PAT.

The following sections describe the operation of these NAT functions.

NOTE NAT also supports TCP load distribution and handles overlapping network addressing; these features are not described in this appendix.

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