Building network automation solutions

Update: This challenge is closed, see the final results (November 4th 2008).

Assuming you have the following configurations on R1 and R2:

hostname R1
!
interface Loopback 0
 ip address 10.0.0.1 255.255.255.255
!
interface Serial 0
 encapsulation ppp
 ip unnumbered Loopback0
 ip ospf 1 area 1
!
router ospf 1

hostname R2
!
interface Serial 0
 encapsulation ppp
 ip address 10.1.2.3 255.255.255.248
 ip ospf 1 area 1
!
router ospf 1

What IP address can you use on the loopback interface of R1 to establish adjacency between R1 and R2? Can you use more than one IP address?

This challenge was triggered by a comment uri wrote on the “OSPF ignores subnet mask mismatch on point-to-point links” post, claiming that you cannot mix numbered and unnumbered interfaces in OSPF.