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 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)4th Octet (binary) 
175.50 00100000 00000000 First usable subnet
175.50 01000000 00000000  
175.50 01100000 00000000  
175.50 10000000 00000000  
175.50 10100000 00000000  
175.50 11000000 00000000 Last usable subnet

Lets look at the first subnet in more detail. 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 Network ID First host Last host 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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