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If you want a thorough detailed explanation of
subnetting I suggest that you visit the following site:
http://www.hardcoremcse.com/ctributor/tcpip3.htm
Here is a simplified explanation of how to get the subnet address ranges from various subnet masks:
YOU HAVE A CLASS B ADDRESS THUS: 131.107.X.Y
SUBNET MASK OF: 255.255.224.0
Q1/. How do I calculate the THIRD & FOURTH OCTETS of the IP ADDRESS?
Look at my Tcp/ip table at the top of this page. With a subnet mask of 224 on a class B address you will see that there are 6 subnets and the bit value is 32. The starting X will be multiples of the bit value 32 with the starting Y being equal to 1 and the finishing X value will be 1 less than the next multiple of 32 and the Y value will be 254.
On a class C address such as 131.107.100.X things change a little. The X starting value is multiples of the bit value plus 1. The X ending value becomes the normal value that you would use above minus 1. The following table depicts this:
Subnet Mask: Class B = 255.255.224.0  Class C = 255.255.255.224 










131.107.32.1  131.107.63.254  131.107.100.33  131.107.100.62 

131.107.64.1  131.107.95.254  131.107.100.65  131.107.100.94 

131.107.96.1  131.107.127.254  131.107.100.97  131.107.100.126 

131.107.128.1  131.107.159.254  131.107.100.129  131.107.100.158 

131.107.160.1  131.107.191.254  131.107.100.161  131.107.100.190 

131.107.192.1  131.107.223.254  131.107.100.193  131.107.100.222 
YOU HAVE A CLASS A ADDRESS THUS: 18.X.Y.Z
SUBNET MASK OF: 255.224.0.0
Q2/. How do I calculate the 2ND, 3RD & 4TH octets?
Now the Class A will follow the same idea. Again there are 6 subnets. The bit value is 32. With the ip address of 18.x.y.z., the starting x will be multiples of the bit value 32, y will be 1 and z will be 1. The ending x will be one less then the next multiple, y will be 255 and z will be 254.























