OSPF IP Unnumbered
router ospf 1 interface Loopback0 ip address 18.104.22.168 255.255.255.255 ip ospf 1 area 0 ! interface Gi0/1 ip unnumbered Loopback0 ip ospf 1 area 0 ! interface Gi0/2 ip unnumbered Loopback0 ip ospf 1 area 0 interface Gi0/3 ip unnumbered Loopback0 ip ospf 1 area 0
However, on some platforms this doesn't work. For example, IOS 15:
R1(config)#interface gi0/1 R1(config-if)# ip unnumbered Loopback0 Point-to-point (non-multi-access) interfaces only
The advantage of
ip unnumbered is that you don't need an IP address on every physical interface, thus you save IP addresses. The disadvantage is that you are unable to ping the physical interface because it doesn't have a unique IPv4 address.
OSPFv3 uses link-local addresses, which is a similar "solution" to
ip unnumbered for IPv4.