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?