Network Addresses

The IP (Internet Protocol) is the "language" of the Internet. Every computer (interface) on an IP network is identified by a unique IP address which facilitates the routing of information. An IP address looks like this: 172.16.94.97. Each of the dot-separated numbers can range from 0 to 255, for a total of over 4 billion possible addresses. Using our example network: Each of the four computers on the inside network has an IP address. The gateway host has two addresses, one for each interface.

The growth of the Internet has created an IP address shortage because there are only so many unique addresses. This is a big problem. The solution to this problem is found in the common use of RFC1918 addresses. These are three ranges of private IP addresses originally set aside for testing purposes, namely, 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255, and 192.168.0.0 to 192.168.255.255. The routers on the Internet are prohibited from forwarding information to any of these addresses making them unreachable, and therefore "invisible."

How does the use of the private IP addresses help relieve the address shortage? Consider our example network. All computers on the internal network, including the inside interface of the gateway host, have been assigned the private addresses. Only the outside interface of the gateway, connected to the outside world, has a "real" (i.e. publicly known and globally unique) IP address. The whole network now uses only a single "real", or public, IP address. The private IP addresses of the internal computers are invisible from the Internet. They can be reused in another network without causing problems, such as addressing ambiguity.

The inside computers can still communicate with the Internet through the gateway, provided that it can translate between private and public IP addresses. For each connection (exchange of information) the gateway must transparently substitute internal private IP addresses with its own public address. To the outside world it appears that all connections originate from the gateway host. The two technologies used to perform this substitution are Network Address Translation (NAT) and the use of Application Proxies. Both will be described in this document.