One of the reasons IP unnumbered is possible is that with only two hosts on any serial link—when the routers are at either end—there is no confusion about which device a packet is originating from or destined for. The following are a few rules that apply to using IP unnumbered:
The serial interface must be a point-to-point link.
You must use the same major network with the same mask on both sides of the WAN link.
You must use different major nets with no subnetting on both sides of the WAN link.
These rules are discussed in more detail in the following sections.
This section provides some background on IP unnumbered and some real-life scenarios of IP unnumbered ...