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.