Subnetting Tutorial
Networking Topics > Layer 3 Routing > 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) | 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. 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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