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Effects of netmasks

class	IP address	Default Netmask	meaning
Class A	16.1.1.1 (16.1 is network portion, 1.1 is host portion)	255.255.0.0	Can now have 254 subnets, 16.1 to 16.254 , each of 65,024 hosts
Class B	132.147.1.1 (132.147.246 is network portion, 1 is host portion)	255.255.255.0	Can now have 254 subnets, 134.147.1 to 254 , each of 254 hosts

Note: Subnets 0 and 255 are not allowed. 255 is the broadcast address and 0 is the same as the netmask.

Class C can be subnetted by dividing up the fourth octet:

(128)	(64)	(32)	(16)	(8)	(4)	(2)	(1)
7	6	5	4	3	2	1	0
0	0	0	0	0	0	0	0
network			host

From the above table, adding bytes 7,6 & 5 (128+64+32) gives 224 which is the subnet i.e. 255.255.255.224 (or binary 11100000 ) . This masks out bytes 7,6 & 5.

For a C class network with an address 221.138.62.0, a subnet of 255.255.255.224 gives six usable subnets as follows:

subnet	hosts	Broadcast address
221.138.62.32	.33-62	221.138.62.63
221.138.62.64	.65-94	221.138.62.95
221.138.62.96	.97-126	221.138.62.127
221.138.62.128	.129-158	221.138.62.159
221.138.62.160	.161-190	221.138.62.191
221.138.62.192	.193-222	221.138.62.223

The three bytes available for the network segment can give the combinations:

• 000
• 001
• 011
• 111
• 010
• 110
• 100
• 101

As 000 and 111 are not allowed, six subnets are available.

A subnet 255.255.255.192 is binary 11000000 (i.e. 128+64=192). The possible combinations are:

• 00 0
• 01 64
• 10 128
• 11 192

00 & 11 are not allowed so there are 2 usable subnets, 64 & 128, giving for example 194.10.68.65-> and 194.10.68.129-> .

Recommended Class C Subnets

Class C bits	mask	Available Subnets	Addresses in each
2	255.255.255.192	2	62
3	255.255.255.224	6	30
4	255.255.255.240	14	14
5	255.255.255.248	30	6
6	255.255.255.252	62	2

How the netmask is used to calculate the network

The logical AND is used to calculate netmask, i.e.

• 0 AND 0 = 0
• 0 AND 1 = 0
• 1 AND 0 = 0
• 1 AND 1 = 1

Taking class B addresses 204.123.16.4 and 204.123.17.4, with a netmask of 255.255.255.0, mask AND address gives 204.123.16.0 and 204.123.17.0, i.e. they are on different networks.

A netmask of 255.255.0.0 gives 204.123.0.0 and 204.123.0.0 so with this netmask, they are on the same network.