How to figure out your default subnet mask
A subnet mask is a 32-bit binary number, very similar to an IP address. And like an IP address, the subnet mask is usually expressed as a set of four octets written in decimal form, with each octet separated by a period.
The subnet mask is used by routers and switches to determine which part of a computer or other device's IP address is the network identifier, and which part is the host identifier. There are three default subnet masks, one for each of the IP classes A, B, or C. The default subnet masks are:
Class A = 255.0.0.0
Class B = 255.255.0.0
Class C = 255.255.255.0
To determine the default subnet mask for your network, all you need to do is see which class your network falls into. A network's class is assigned based on the first octet (the set of digits before the first period) of the IP address. For example, to find out which class the network 220.127.116.11 belongs to, you would see where "123" fits into the following table:
First Octet: Class:
1-126 = Class A
128-191 = Class B
192-223 = Class C
So for the example above, 18.104.22.168 is a Class A network, which means it would have a default subnet mask of 255.0.0.0.
Most home and office networks will be Class C networks, so your default subnet mask will typically be (you guessed it) 255.255.255.0. Not all networks use the default subnets, but for those that do, figuring it out is easy once you know which class your network fits into.
on Jun 23, 2010 | PC Desktops