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Special Unicast IPv4 Addresses

In addition to the addresses that cannot be assigned to hosts, special addresses can also be assigned to hosts but with restrictions on how those hosts can interact within the network. These special addresses include the following:

■   Default route

■   Loopback address

■   Link-local address

■   Test-net addresses

The following sections summarize these special IPv4 addresses.

Default Route

The IPv4 default route is 0.0.0.0. This default route is a “catch all” route to route packets when a more specific route is not available. The use of this address also reserves all addresses in the 0.0.0.0 /8 address block (0.0.0.0–0.255.255.255).

Loopback

Another reserved address block is 127.0.0.0 /8 (127.0.0.0 to 127.255.255.255). This is reserved in the IPv4 hosts for the 127.0.0.1 loopback. The loopback is a special address that hosts use to direct traffic to themselves. The loopback address creates a shortcut method for TCP/IP applications and services that run on the same device to communicate with one another. By using the loopback address instead of the assigned IPv4 host address, two serv-ices on the same host can bypass the lower layers of the TCP/IP stack. You can also ping the loopback address to test the configuration of TCP/IP on the local host.

Although only the single 127.0.0.1 address is used, address block 127.0.0.0 /8 (127.0.0.0 to 127.255.255.255) is reserved. Any address within this block will loop back within the local host. No address within this block should ever appear on any network.

Link-Local Addresses

IPv4 addresses in the 169.254.0.0 /16 address blocks (169.254.0.0 to 169.254.255.255) are designated as link-local addresses. These addresses can be automatically assigned to the local host by the operating system in environments where no IP configuration is available. These might be used in a small peer-to-peer network or for a host that could not automati-cally obtain an address from a Dynamic Host Configuration Protocol (DHCP) server.

Communication using IPv4 link-local addresses is only suitable for communication with other devices connected to the same network. A host must not send a packet with an IPv4 link-local destination address to any router for forwarding and should set the IPv4 TTL for these packets to 1.