I am not going to go over in details on the hoods of PPP (another post will be written for this purpose).
A deep understanding of Point-to-Point WAN networks isn’t required as we will barely scratch that surface when dealing with the “unnumbered IP command”.
So what is the unnumbered IP command?
The unnumbered IP command enables you to enable IP routing/processing on an interface by assigning it an IP already in used of another interface. That is to say, the unnumbered IP command enables you to have 2 interfaces, a multi-access interface and a point to point interface (often a serial port) to share the same IP address.
–> Now why would you do that? Simple, to save IPs and avoid in many cases to use VLSM if not needed.
You see when dealing with a PPP network, which simply involves 2 WAN interfaces, you would only be needing a /30 subnet prefix (yes! you could also use a /31 and suppress the broadcast)… however, when generally assigned a /16 or even a /24 email address, you would be loosing IPs when subnetting your network. You can of course use VLSM to avoid this, or you can simply use the unnumbered IP command :-)
How do I configure a serial interface with the unnumbered IP command?
I HIGHLY recommend using a loopback interface since the logical interface would always be in the UP/UP state as long as of course that the router itself is up :-).
ip address 192.168.1.10 255.255.255.240
ip unnumbered loopback0
And that’s it…
Please note that when using the unnumbered IP command, the interface will be unavailable for remote testing and management