IP Subnetting and Variable Length Subnet Masks (VLSMs)
A. A /25 mask is 255.255.255.128. Used with a Class B network, the third and fourth octets are used for subnetting with a total of 9 subnet bits, 8 bits in the third octet and 1 bit in the fourth octet. Since there is only 1 bit in the fourth octet, the bit is either off or on—which is a value of
or 128. The host in the question is in the 0 subnet, which has a broadcast address of 127 since 128 is the next subnet.
C. Start with 255.255.255.0. This provides 256 subnets, each with 254 hosts. Move the subnet bits right if you need more subnets; move them left if you need more hosts. Since we need more hosts, we’re going to take away subnet bits. The next mask then is 255.255.254.0, which pro- vides 7 subnet bits and 9 host bits, or 128 subnets, each with 510 hosts.
C. The default gateway is the subnet address and not a valid host ID.
E. A Class C subnet mask of 255.255.255.224 is 3 bits on and 5 bits off (11100000) and pro- vides 8 subnets, each with 30 hosts. However, if the command ip subnet-zero is not used, then only 6 subnets would be available for use.
E. A Class B network ID with a /22 mask is 255.255.252.0, with a block size of 4 in the third octet. The network address in the question is in subnet 172.16.16.0 with a broadcast address of 172.16.19.255. Only answer E even has the correct subnet mask listed, and
is a valid host.
C. A Class B /20 mask is 255.255.240.0, which is 4 subnet bits and 12 host bits. Eight host bits is 256 (minus 2 is 254 hosts). Nine hosts bits is 256 + 256 = 512 (minus 2 is 510 hosts per subnet). Ten host bits is 512 + 512 = 1024 (minus 2 is 1022 hosts per subnet). Eleven hosts bits is 1024 + 1024 = 2048 (minus 2 is 2046 hosts per subnet). Twelve host bits is 2048 + 2048 = 4096 (minus 2 is 4094 hosts per subnet).
20. B, C, D. This question only looks somewhat difficult because Class A, B, and C network addresses are listed. But they are only there to confuse you. What is a /27? 255.255.255.224. This is the mask you will use regardless of the class of address. Take out all the subnet and broadcast addresses and you’re left with just the valid hosts.