What each octet means – subdividing a network into multiple subnets

The Network Part vs. the Host Part

• 192.168.0 –> Network Part
• .0  –> Host Part

A subnet mask of 255.255.255.0 means that the first three octets of the address will be used for the network, and thus our network number is 192.168.1.
This means we can have 254 computers on this network, because the fourth octet is not being used by the network portion of the address

Reserved address ranges for private (non-routed) addresses

10.0.0.0 -> 10.255.255.255
172.16.0.0 -> 172.31.255.255
192.168.0.0 -> 192.168.255.255

Dividing the network into multiple subnetworks

There are two methods for dividing your network into multiple sub networks; One is to simply change your network numbers keeping the same subnet mask. The other is to subnet your network into smaller sub networks.

Option 1 – Keeping the same mask:

Your network could be divided into two or more networks by changing the network portion of the address such as 10.0.1 and 10.0.2 and keeping the same subnet mask.

Example:
192.168.1.0/255.255.255.0
192.168.2.0/255.255.255.0

Doing this would give you two separate networks with 254 hosts per network. This is a very common method of dealing with multiple networks. However, back in the good old days you had to pay for every IP address you used, and if you had 25 computers on your network you probably would not want to pay for 254 addresses! The answer to the problem is…subnetting.

Option 2 – Subnetting a network

Subnetting is when you use bits from the host portion of your address as part of your network number. This let’s you subdivide your network at the cost of host addresses, which is great if you’re paying for every host IP address. It will save you money because you pay for fewer TCP/IP addresses. Confused? Here is where understanding binary is important.

Lets look at a new subnet mask:

255.255.255.224

As you can see in the fourth octet, some of the host portion of this subnet mask is now being used for part of the network address. Which means we are now using some of the binary bits in the fourth octet for our network numbers, and that gives us fewer hosts than our old mask (which gave us 254), but gives us more networks (which is why we call it subnetting).