CCENT/CCNA ICND1 100-101 Official Cert Guide

Mask Analysis Problem 4: Answer

Address 10.22.55.87 is in Class A network 10.0.0.0, meaning that 8 network bits exist. The prefix format mask of /30 lets you calculate the number of host bits as 32 – prefix length (in this case, 32 – 30 = 2). This leaves 22 subnet bits (32 – 8 network bits – 2 host bits = 22 subnet bits). The number of subnets in this Class A network, using mask 255.255.255.252, is 2²² = 4,194,304. The number of hosts per subnet is 2² – 2 = 2. (Note that this mask is popularly used on serial links, which need only two IP addresses in a subnet.)