Which of the following network commands following the command router ospf 1, tells the router to start using OSPF on interfaces whose IP addresses are 20.1.20.1, 20.1.30.1, and 20.1.40.1?

(A) network 20.0.0.1 0.0.255.255 area 0
(B) network 20.0.0.1 0.0.0.255 area 0
(C) network 20.0.0.0 255.0.0.0 area 0
(D) network 20.0.0.0 0.255.255.255 area 0

network 20.0.0.0 0.255.255.255 area 0 is the correct answer.

• The network command network 20.0.0.0 0.255.255.255 area 0 matches all interfaces whose first octet is 20.
• The network command network 20.0.0.0 255.0.0.0 area 0 means all addresses that end with 0.0.0 (wildcard mask 255.0.0.0). In this case, is a wrong answer.
• The network command network 20.0.0.1 0.0.0.255 area 0 means all addresses that begin with 20.0.0 (wildcard mask 0.0.0.255). In this case, is a wrong answer.
• The network command network 20.0.0.1 0.0.255.255 area 0 means all addresses that begin with 20.0 (wildcard mask 0.0.255.255). In this case, is a wrong answer.