Subnetting, The Struggle for Understanding

Subnetting, The Struggle for Understanding John Skyers HBC Regional Academy London SE17 1JE United Kingdom

Subnetting is the logical creation of new IP networks, from one IP network address.

In order to subnet a network, you need to have a good understanding of IP addresses

So lets review IP addresses and roll straight through into subnetting

IP addresses are: • Assigned by American Registry for Internet Numbers (ARIN). • Four 8 bit binary numbers connected by periods. Each number division is called an octet. These numbers are displayed in decimal form eg. 210.10.4.20 • Divided into classes dependent on the number within the first octet.

The class of address? Class A 0-127 (0 Default; 127 Loop back test) Class B 128-191 Class C 192-223 Class D 224-239 (Multicasting) Class E 240-254 (Experimental; Research)

Each class of address has a different code. The code displays the network and host octets of each address and its corresponding subnet mask • Class A N.H.H.H • Class B N.N.H.H • Class C N.N.N.H

“Subnet Mask?”

A subnet mask allows computers to distinguish between the network, sub-network and host bits of a given IP address

Default Subnet Masks are used when subnetting is not occurring. The defaults are: • 255.0.0.0 for Class A addresses • 255.255.0.0 for Class B addresses and • 255.255.255.0 for Class C addresses.

Now lets subnet!

Given the Class C network address 210.50.4.0, create 6 subnets.

Number of bits to borrow • First step, find out how many bits you need to borrow for subnet. (Note: You can only borrow from host octets; you must borrow at least 2 bits and you must leave at least 2 bits)

Number of bits to borrow • By using a constant (2), 2³ = 8, we discover that 3 bits need to be borrowed (Answer must be <=6 when you minus 2, I will explain later).

Subnet Number • Now, by adding the 3 (number of bits borrowed) high order binary numbers (128 +64 +32) we get the number 224. This number is known as the subnet number.

Custom subnet mask • The Default Subnet Mask is only used when subnetting is not occurring. • The moment you start subnetting you have to create a Custom Subnet Mask. • This is simply a matter of dropping the subnet number into the first host octet of the Default Subnet Mask. • This creates a Custom Subnet Mask for our present example: 255.255.255.224

Subnet range of addresses • If we minus the subnet number (224) from another constant, 256 (magic number), we get the number 32 (the first useable subnet address). • We then add up in multiples of 32 until we have 6 IP address ranges, which look like….

THIS! • Subnet (1) 32 – 63 • Subnet (2) 64 – 95 • Subnet (3) 96 - 127 • Subnet (4) 128 – 159 • Subnet (5) 160 – 191 • Subnet (6) 192 - 223

The first number in range is theSubnet Address • Subnet (1) 32 – 63 • Subnet (2) 64 – 95 • Subnet (3) 96 - 127 • Subnet (4) 128 – 159 • Subnet (5) 160 – 191 • Subnet (6) 192 - 223

While the last number in range is the subnet’s Broadcast Address • Subnet (1) 32 – 63 • Subnet (2) 64 – 95 • Subnet (3) 96 - 127 • Subnet (4) 128 – 159 • Subnet (5) 160 – 191 • Subnet (6) 192 - 223

The subnet addresses and broadcast addresses cannot be used for hosts or subnets therefore for every range, two addresses cannot be used by devices and by subnets (this was why we minus 2 in our previous subnet calculation) • Subnet (1) 32 (Useable Host Addresses) 63 • Subnet (2) 64 ((Useable Host Addresses) 95 • Subnet (3) 96 (Useable Host Addresses) 127 • Subnet (4) 128 (Useable Host Addresses) 159 • Subnet (5) 160 (Useable Host Addresses) 191 • Subnet (6) 192 (Useable Host Addresses) 223

Formula’s you need to know!

Subnets • Total subnets = 2^(Bits used for subnet) • Total useable subnets = 2^(Bits used for subnet)-2

Hosts • Total hosts = 2^(Bits used for hosts) • Total useable hosts = 2^(Bits used for hosts) - 2

Now let me let you in on a secret. The key to mastering IP addressing and Subnetting is……

PRACTICE! PRACTICE! PRACTICE! Good luck on your CNAP at

Subnetting, The Struggle for Understanding