In NAT terminology, the inside network is the set of networks that is subject to translation. The outside network refers to all other networks.

When using NAT, IPv4 addresses have different designations based on whether they are on the private network, or on the public network (Internet), and whether the traffic is incoming or outgoing.

NAT includes four types of addresses:

When determining which type of address is used, it is important to remember that NAT terminology is always applied from the perspective of the device with the translated address:

NAT also uses the concept of local or global with respect to addresses:

In the figure, PC1 has an inside local address of From the perspective of PC1, the web server has an outside address of When packets are sent from PC1 to the global address of the web server, the inside local address of PC1 is translated to (inside global address). The address of the outside device is not typically translated, because that address is usually a public IPv4 address.

Notice that PC1 has different local and global addresses, whereas the web server has the same public IPv4 address for both. From the perspective of the web server, traffic originating from PC1 appears to have come from, the inside global address.

The NAT router, R2 in the figure, is the demarcation point between the inside and outside networks and as between local and global addresses.