Subnet Calculator PDF Print E-mail
Written by David Noel-Davies   

Subnet Calculator

What is a Subnet Calculator good for? Well for 99% of Internet users it is of no importance. So if you don't know what a subnet is this is probably not the Calculator for you.

A Internet address is a set of numbers like A Internet address is actually a 32bit Integer. The address is divided into two parts, the network number and the host number. The net mask is used to show the Network number. In the old days there where only three net masks and for A,B and C class networks. With the increasing number of hosts on the Internet a more effective scheme is used called Classless Inter-Domain Routing (CIDR). With CIDR you decide how many bits are being used for the network part. A old Class C address ( is in CIDR notation /24 (i.e. the first 24 bits of the address is used for the network number) The Subnet calculator start of with mask /25. If you are using 25 bits, your subnet mask is (check the lower part of the tool), you can divide a old Class C network into two parts. Now slide the network address slider past 127, you will see that the Network Address changed from to You can also see that the broadcast address changed. If you want to make a list of all possible CIDR networks from the chosen mask/address press the Generate subnet list. Hopefully this will give you an idea of how sub netting works

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