Loopback Address

Another special subnet we use is /32. It is not immediately obvious how to use a network which has no nodes! One very useful thing we have in large routed networks is a loopback address.

Imagine a network router, a device which routes packets from network to network at layer 3. We will call it Router1.

This router will have an interface in each network it routes between; it will therefore have many IP addresses. The router in the diagram has three such interfaces.

• 192.168.0.1/24

• 192.168.1.1/24

• 192.168.2.1/24

It can be very useful to have a single IP address which identifies this router and which we can access. It is normal practice to create a loopback address which uniquely identifies this router. The loopback address (L0) is a node address and a network address, all in one.

In this case we use 192.168.255.1/32 and we can access the router with this address from any network.

Router2 might use the loopback address 192.168.255.2/32 and so on.

We will see in later sessions that we can use dynamic routing protocols to pass routing information around a network. Loopback addresses are a critical component of many of these protocols.