2.7 Using IP Unnumbered
2.7.4 Host routes
A host route refers to a route whose associated mask has all 32 bits set to 1 --- 255.255.255.255. For an address and mask such as this, there can be only one host.

One common cause of host routes is a route advertisement that has bits set in the range that the receiving router interprets as outside of the subnet mask. In this case, the router compared the advertisement received from an adjacent router with the mask of the interface it was received over and found a mismatch. This is how RIP V1 and IGRP determine the mask to apply to advertisements they receive.

Another common cause of host routes is a routing update explicitly advertised with a mask of 255.255.255.255. Routing protocols such as OSPF and EIGRP include the masks associated with the routes in their routing updates. Host routes are also often manually configured to resolve routing problems related to configuring dial-on-demand routing (DDR).

Sending Routing Updates

The output of debug ip rip from RouterA shows RouterA advertising the summarized major network of 168.71.0.0. (see RouterA#debug ip rip command output).

The output of debug ip rip from RouterC shows RouterC advertising the summarized major network of 168.71.0.0. (see RouterC#debug ip rip command output).

Although both routers are advertising the summarized major net of 168.71.0.0 to each other, neither router will install the route while using RIP because they have local connections to the 168.71.0.0 major net.

Pinging the Interfaces

In the output, which is from RouterA pinging 168.71.8.1, you can see that the pings are successful. (see RouterA#ping 168.71.8.1 command output).

In the output, which is from RouterC pinging 168.71.5.1, you can see that the pings are successful. (see RouterC#ping 168.71.5.1 command output)

The fact that the pings are successful is the final proof that IP unnumbered has been configured correctly. Strictly speaking, this step is unnecessary as long as the results from the show ip route command indicate that proper routes have been installed. However, it is always better to double-check proper configuration by pinging the addresses.

The next section is a scenario about what happens when IP unnumbered has been misconfigured.