NAT Terminology

NAT uses the terms inside and outside, and local and global, as shown in Figure D-1 and defined in Table D-1.

Figure D-1 Network Address Translation Is Used to Translate Addresses Between the Inside and Outside Networks




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Host B

Inside Local IP Address

Inside Global IP Address

NOTE Recall that the IP addresses shown in the examples in this book are private addresses. In practice, public addresses would be used on the Internet.

Table D-1 NAT Terminology



Inside local IP address (A)

The IP address assigned to a host on the inside network. The address is typically an RFC 1918 (Address Allocation for Private Internet Space) address.

Inside global IP address (B)

A globally unique IP address (typically assigned by an ISP) that represents one or more inside local IP addresses to the outside world.

Outside global IP address (C)

The IP address assigned to a host on the outside network by its owner. The address is globally unique.

Outside local IP address (not shown)

The IP address of an outside host as it appears to the inside network. The address is typically allocated from address space that is routable on the inside, usually from the RFC 1918 address space.

Simple translation entry

A translation entry that maps one IP address to another. The NAT table in Figure D-1 shows this type of entry.

Extended translation entry (not shown)

A translation entry that maps one IP address and port pair to another.

NOTE The terms inside, outside, local, and global in Table D-1 are relative references in the end-to-end transmission path between devices, depending on the viewpoint of the end station. For example, an inside local address from a sender's point of view is an outside local address from a receiver's point of view.

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