Internet Core Protocols: The Definitive Guide

Internet-Legal Versus Private Addressing

Although the pool of IP addresses is somewhat limited, most companies have no problems obtaining them. However, many organizations have already installed TCP/IP products on their internal networks without obtaining “legal” addresses from the proper sources. Sometimes these addresses come from example books or are simply picked at random (several firms use networks numbered 1.2.3, for example).

These addresses are not legal, and will not be usable when these organizations attempt to connect to the Internet. These firms will eventually have to reassign Internet-legal IP addresses to all of the devices on their networks, or they will have to invest in address translation gateways that re-write outbound IP packets so that they appear to be coming from an Internet-accessible host.

Even if an address translation gateway is installed on the network, these firms will never be able to communicate with the sites that are the registered owners of the IP addresses in use on the local network. For example, if you choose to use the Class A address block of 36 on your corporate network, then your users will never be able to access the computers at Stanford University, which is the registered owner of that particular address block. Any attempt to connect to a host at 36.x.x.x would be interpreted by the local routers as a request for a local system, so packets would never leave your local network.

Not all firms have the luxury of using Internet-legal addresses ...