Which single OSPF network statement will correctly include all the interfaces on a device whose IP addresses only begin with a 10.10.8, 10.10.9, 10.10.10, or 10.10.11?

A) network 10.10.8.0 0.0.7.255 area 0
B) network 10.10.0.0 0.0.0.255 area 0
C) network 10.10.8.0 0.0.3.255 area 0
D) network 10.10.0.0 0.0.7.255 area 0

C
Explanation: C) The concept of a wildcard mask often confuses those new to Cisco networking and should be practiced until it is clear how it works. For this example, 10.10.8.0 0.0.3.255 will include all interfaces on a device with IP addresses that begin with 10.10.8, 10.10.9, 10.10.10, and 10.10.11. The 0.0.3.255 address works out to a subnet mask of 255.255.252.0, so the subnet 10.10.8.0 255.255.252.0 would include these same address ranges. All you have to do is invert the subnet mask: 255.255.252.0 = 11111111 11111111 11111100 00000000 inverted = 00000000 00000000 000000111111111 = 0.0.3.255.