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B-6 Reference Guide
As you can see, subnet masks that allocate one, seven, or eight
bits to subnets are useless for a Class C network. This is because
binary host addresses or subnet addresses that are composed of
all zeros or all ones are reserved for broadcasting (see
“Broadcasts” on page B-16). Class A or Class B networks, on the
other hand, would still have many host numbers available if the
network manager chose a subnet mask that allocated seven or
eight bits to subnets.
1. Decide on a subnet strategy
Your 25 IP hosts are arranged as follows: 10 on the third floor, eight
on the fourth floor, and seven on the fifth floor. Since you will need
at least ten host addresses per subnet, the preceding table
indicates that you must choose a subnet mask that allocates four or
fewer bits to the subnet address. You decide to use a subnet mask
Subnetting options for a Class C IP network
Subnet mask chosen
Number of bits for
subnet number
Number of
subnets possible
Number of
hosts possible
on each subnet
11111111.11111111.11111111.10000000
or 255.255.255.128
1 0 126
11111111.11111111.11111111.11000000
or 255.255.255.192
2262
11111111.11111111.11111111.11100000
or 255.255.255.224
3630
11111111.11111111.11111111.11110000
or 255.255.255.240
41414
11111111.11111111.11111111.11111000
or 255.255.255.248
5306
11111111.11111111.11111111.11111100
or 255.255.255.252
6622
11111111.11111111.11111111.11111110
or 255.255.255.254
7 126 0
11111111.11111111.11111111.11111111
or 255.255.255.255
8 254 0