IP Services — NAT


Inside Local Address (inside the enterprise): normal inside address (from private address pools). Addresses not advertised to the outside.

Inside Global Address (inside the enterprise): legitimate IP address representing one or more inside local IP address to the outside world. “Real” address that gets translated into one or more outside local addresses. Addresses by which inside devices are known to the outside.

Outside Local Address (In the public Internet; or outside the enterprise network): Part of the enterprise IP address space; typically a private IP address. Addresses by which outside devices are known to the inside.

Outside Global Address (public internet or outside enterprise): Part of the public address space. A legitimate IP address assigned to a host on the outside global network by the host’s owner. Globally routable address. These addresses are not advertised inside.

Outside Local addresses are mapped to Outside global addresses.

So for example, Host A’s internal IP is This is the IL IP address. When a packet is sent out, the source changes to, the IG IP address. Host B’s IP address which is on the outside is, this is the Outside Global IP address, not known to Host A. When Host A sends a packet to Host B, it uses the IP address:, which is the OL IP address. When this IP address goes through the router, it changes from OL to OG ( (as the destination address).


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