The "network 192.168.12.64 0.0.0.63 equals to network 192.168.12.64/26. This network has: Increment: 64 (/26= 1111 1111.1111 1111.1111 1111.1100 0000) + Network address:192.168.12.64 Broadcast address: 192.168.12.127 Therefore all interface in the range of this network will join OSPF.