Internet Core Protocols: The Definitive Guide

How IP finds remote hosts and networks

Every IP device—regardless of the function it serves—must have an IP address for every network that it is connected to. Most systems (such as PCs) only have a single network connection, and therefore only have a single IP address. But devices that have multiple network interfaces (such as routers or high-load devices like file servers) must have a dedicated IP address for every network connection.

When the IP protocols are loaded into memory, an inventory is taken of the available interfaces, and a map is built showing what networks the system is attached to. This map is called a routing table: it stores information such as the networks that the node is connected to and the IP address of the network interface connected to that network.

If a device only has a single interface, then there will be only one entry in the routing table, showing the local network and the IP address of the system’s own network interface. But if a device is connected to multiple networks—or if it is connected to the same network several times—then there will be multiple entries in the routing table.

Tip

In reality, just about every IP device also has a “loopback” network, used for testing and debugging purposes. The loopback network is always numbered 127.0.0.0, while the loopback interface always has the IP address of 127.0.0.1. This means that routing tables will generally show at least two entries: one for the physical connection and one for the loopback network. ...