Subnetting Tutorial

The process of subnetting involves dividing a network up into smaller networks called subnets or subnetworks.

To create these additional networks we use a subnet mask. The subnet mask simply determines which portion of the IP address belongs to the host.

The subnet mask does not alter the class of the IP address, it simply "borrows" bits from the host portion and uses these to create subnets. This naturally reduces the maximum number of hosts your network can have, because you are using some of your host bits for your subnet bits.

The subnet mask can borrow as many bits as you like, the more bits you borrow, the more subnets you can create. Bear in mind though, that increasing the number of subnets decreases the number of hosts that each subnet can have.

You can calculate the number of bits to borrow using the following equation.

2ⁿ - 2 = number of subnets

n = number of host or node bits

Notice that this equation is very similar to the number of available hosts equation.

Subnetting Example

Dotted Decimal IP	175	50	0	0
Binary IP	10101111	00110100	00000000	00000000
Standard subnet mask for Class B	255	255	0	0
Binary standard subnet mask	11111111	11111111	00000000	00000000
Subnetted mask	255	255	224	0
Binary subnetted mask	11111111	11111111	11100000	00000000

As you can see form the example above if we use the subnet mask of 255.255.224.0 on a class B IP address then we will be "borrowing" 3 host bits to use for subnetting. From our equation we can calculate that this will create 2³-2 = 6 useable subnets and this will leave us with 13 host bits for each subnet, which will provide us with 2¹³-2=8190 hosts per subnet.

Subnet Ranges

1st 2 Octets (dotted decimal)	3rd Octet (binary)
175.50	00100000	First usable subnet
175.50	01000000
175.50	01100000
175.50	10000000
175.50	10100000
175.50	11000000	Last usable subnet

Lets look at the first subnet in more detail. 175.50.32.0. This address is the "wire" or "network" address of the subnet.

First Subnet Range

1st 2 Octets (dotted decimal)	3rd Octet (binary)	4th Octet (binary)
175.50	00100000	00000000	Network ID
175.50	00100000	00000001	First host
175.50	00111111	11111110	Last host
175.50	00111111	11111111	Broadcast address

First Subnet Range in Dotted Decimal

175.50.32.0	Network ID
175.50.32.1	First host
175.50.63.254	Last host
175.50.63.255	Broadcast host

As you can see from the above example, it's much easier to work out subnetting in binary than decimal format.

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