Netopia 435 Reference Manual page 228

C-6 Netopia ISDN Router Reference Guide
Subnet mask chosen
11111111.11111111.11111111.10000000
or 255.255.255.128
11111111.11111111.11111111.11000000
or 255.255.255.192
11111111.11111111.11111111.11100000
or 255.255.255.224
11111111.11111111.11111111.11110000
or 255.255.255.240
11111111.11111111.11111111.11111000
or 255.255.255.248
11111111.11111111.11111111.11111100
or 255.255.255.252
11111111.11111111.11111111.11111110
or 255.255.255.254
11111111.11111111.11111111.11111111
or 255.255.255.255
Subnetting options for a Class C IP network
Number of bits for
subnet number
1
2
3
4
5
6
7
8
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
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
Number of
subnets possible
0
2
6
14
30
62
126
254
C-16). Class A or Class B networks, on the
Number of
hosts possible
on each subnet
126
62
30
14
6
2
0
0