An Acronym for Network Address Translation.

NAT is an evil scheme to change IPs as they travel through a gateway. A common special case is "Masquerading", where multiple machines appear to have the gateway's IP address, which is necessary if you have multiple machines trying to communicate with remote hosts on the InterNet but the ISP only gave you one IP address.

If you are trapped behind NAT and require a realworld IP, consider using Teredo to get yourself a realworld IPv6 address.

See RFC:1631, NAPT, Teredo, STUN?.

There are multiple types of NAT (from RFC:3489)

Full cone

When a packet is sent out of the NAT device and the NAT device uses port say 1234 then any packet arriving at the NAT device for port 1234 will be sent back through to the host inside.

Restricted Cone

This is like Full cone, except that the internal host must have spoken to the remote host at some point in the past.

Port Restricted Cone

This is like Restricted Core but they must have spoken to the same IP:Port before.

Symmetric NAT

The worst kind of NAT. This creates a seperate port for every remote host:port pair, and thus doesn't work with Teredo or STUN?.