Let us consider serial links connecting LANs. For such a link, a subnetwork of at least four IP addresses is needed (a network address, a network broadcast, and two addresses for network interfaces on routers).
Figure 6.6: LAN connected via serial links
It is obvious from Figure 6.6 that besides the three IP address intervals for local networks, other addresses for serial link networks will be necessary. It is clear at first glance, that not using another network address for serial links would be very efficient.
Current routers are able to create an unnumbered interface on point-to-point links adjacent routers act as a single virtual router. Each physical router represents one half of this virtual router. The virtual router has two interfaces only—one per each LAN.
Figure 6.7: Unnumbered networks
There is no need then to waste IP addresses for serial links.