Special-Purpose IPv4 Addresses

Many IPv4 addresses are reserved for special-purpose. For example the network address and broadcast address that we cannot assign to hosts on the network. But some addresses are reserved as special addresses that we can allot to hosts in the network with some limitations on how those hosts can interact within the network.

Loopback addresses

The IP address 127.0.0.0/8  is a special-purpose IPv4 address called localhost or loopback address. 127.0.0.1 is usually used as “the” loopback address, but the rest of the block should loopback as well, meaning that the block not used for anything. For example, most Cisco switches used 127.0.0.xx IPs to listen for attached cards and modules, so at least some of other addresses are in use.

The IP address 127.0.0.1 is reserved for use on personal computer loopback testing in most cases. The host used the loopback IP address to direct traffic to itself. For example, we can use the loopback IP address to test if the TCP/IP configuration is operational or not.  Network software and utilities also use 127.0.0.1 to access a local computer’s TCP/IP network resources.

Messages destined to loopback addresses do not reach outside to the local area network (LAN) but instead are automatically re-routed by the computer’s own network adapter back to the receiving end of the TCP/IP stack. All computers use this IP address as their own IP address but it doesn’t let them communicate with other devices as a real IP address does. The Computer uses this special IP address in special circumstances. For example, a web server running on a computer can point to 127.0.0.1 so that the pages can be run locally and tested before its deployed. This is unlike a regular IP address that is used to transfer files to and from other networked devices.

Link-Local addresses

Link-local addresses mostly assigned automatically using operating system procedures but we can also assign Link-Local Address manually. Normally Link-Local addresses only used when no mechanism of address configuration exists, such as DHCP, or when another configuration method has failed. The addresses are also known as the Automatic Private IP Addressing (APIPA).

It can be used to communicate two hosts, which are there on the same link when no other IP address is specified. In other words, when the system is starting up and the operating system tries to configure an address on its interface through various methods like Dynamic Host Configuration Protocol (DHCP) and Manual Configuration.

If Operating System fails to configure an IP address on the interface through any of the IP address configuration methods, then it configures an address on the interface from the link-local pool which is 169.254.0.0/16 in IPv4 address space

TEST-NET addresses

These addresses are set aside for teaching and learning purposes. Address prefixes listed in Test-Net addresses along with other Special-Purpose Address Registry are not guaranteed any refutability in any particular local or global network. The following ranges are reserved for special purpose In RFC-5737.

192.0.2.0/24        TEST-NET-1                RFC 5737

198.51.100.0/24     TEST-NET-2                 RFC 5737

203.0.113.0/24      TEST-NET-3                 RFC 5737

Default Router Address

The default route also is known as the gateway of last resort used to forward packets whose destination address not match any route in the routing table. The CIDR notation for a default route is 0.0.0.0/0. So when both the host and network portion along with the prefix length is zero a default route is a shortest possible match. So, when a packet does not match any route in the routing table it will match a default route, the shortest possible route if it exists in the routing table.

A default route is very useful in networking where learning all the routes is not desirable such as in the case of stub networks. A default route is greatly useful when a router is connected to the Internet. Without a default route, the router must need the routing entry for all networks on the Internet. The network on the internet is in numbers of several hundred thousand. However, we can configure it with single route.