User manual

B-4 Addressing Scheme
s
Subnets of Class C networks
Since Class C networks are by far the most common, we will
take a closer look at subnetting in a Class C network. The
following table is a listing of all possible values for the last octet
(byte) in a Class C subnet mask. Hosts/Subnet
Mask Binary Subnets Hosts/Subnet
128
10000000 0 0
192
11000000 2 62
224
11100000 6 30
240
11110000 14 14
248
11111000 30 6
252
11111100 62 2
254
11111110 126 0
Class C subnet masks
Two important things must be noticed about the address
divisions created by a subnet mask.
1.
RFC 950 requires that the first and last subnet created by a
mask are reserved. So, the number of usable subnets is
always 2 less than the number of divisions created. This
makes 128 an unusable netmask because it has no legal
subnets!
2.
The first and last host address in each subnet are also
reserved (see Reserved Addresses below). This means 254 is
also an unusable subnet mask because there are no legal
host addresses!