Hello,
Correct, with IP Unnumbered feature configured, your router shares an IP address among several interfaces. However, you have to note that only the IP address (essentially with a /32 mask) is shared, not the entire subnet of the "master" interface.
The idea behind the IP Unnumbered is that the point-to-point interfaces represent transit links, i.e. interconnections through which data are routed but where usually no end hosts (i.e. sources or destinations) are located. For pure routing purposes, such links do not even need IP addresses at all because there is nobody to be addressed on that link, as both the sources and destinations are somewhere off that link. However, on Cisco devices, if you do not configure an interface with an IP address then the IP protocol processing is deactivated on that interface, so it is incapable of sending/receiving IP packets. In addition, if a routing protocol is to be used on that interface, it must send its messages using some kind of a source IP address, and again, without an IP address on that interface, it is not possible.
So the IP Unnumbered feature is a compromise here. It allows an interface to have an IP address alright, but at the same time, instead of using a separate IP address space, the IP Unnumbered simply shares an IP address from another interface. Why is that allowed? Because, as I indicated earlier, there is no globally interesting source or destination located on a transit point-to-point link, so there is no point in having an unique IP address on such an interface.
The routing with IP Unnumbered interfaces is fairly simple, as the routing table will declare all networks reachable behind an unnumbered link simply with the outgoing interface (and optionally with the IP address of the opposite router).
You may be interested in reading the following article in detail:
http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094e8d.shtml
Best regards,
Peter
