Overloading Inside Global Addresses

Overloading Inside Global Addresses
You can reduce the number of IP addresses in the inside global IP address pool by allowing the NAT
border router to use a single inside global IP address for many inside local IP addresses; this is called
PAT or overloading. When NAT overloading is enabled, the router maintains additional information
in the NAT table to keep track of the layer 4 protocol information. When multiple inside local
IP addresses map to one inside global IP address, NAT uses the protocol and TCP/UDP port number
of each inside host to make a unique and distinguishable inside global IP address/port combination,
or socket. For the rest of this chapter, the word address will imply socket when referring to PAT.
Because you are using a pool of IP addresses, the pool can contain more than one IP address.
This allows a very large number of hosts’ inside local addresses to be translated to a small pool
of inside global IP addresses when using overloading.
Figure 3.3 shows the NAT operation when one inside global IP address represents multiple
inside local IP addresses. The TCP port number represents the unique portion of the inside
global IP address that makes it capable of distinguishing between the two local IP addresses on
the inside of the network.