A subnet is to devide your network in subnetworks, this usually is done the the Network Administrator, changing your subnet means you are entering in another network, or your computer will not fit in any subnetworks.
Calculating a subnet(for class C):
Lets say this is your netwotrk 192.168.0.0/28, what does this mean the network size is 28.lets change it into Bits.
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192.168.0.0 = 11000000.10101000.00000000.00000000
28 = 11111111.11111111.11111111.11110000 = 255.255.255.11110000 = 255.255.255.240
As you saw in the last value there is only 4 bits of 1 remaining, the number of subnets will be 2^4(2 at power of 4) = 16, 256/16 = 16(where 2 is the network first and last addresses , network and broadcast).
So the networks will be:
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192.168.0.1-192.168.0.14
192.168.0.17-192.168.0.30
...
For a class B:
Lets say this is your netwotrk 192.168.0.0/28, what does this mean the network size is 28.lets change it into Bits.
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192.168.0.0 = 11000000.10101000.00000000.00000000
28 = 11111111.11111111.11111111.11110000 = 255.255.11111111.11110000 = 255.255.255.240
The last 2 values will be 12 bits so 2^12 = 4096 which means 4096 subnetworks, a subnetwork size will be 256*256/4096 = 65536/4096 = 16(where 2 is the network first and last addresses , network and broadcast)
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192.168.0.1-192.168.0.14
192.168.0.17-192.168.0.30
...
http://en.wikipedia.org/wiki/SubnetworkAnother way to find your subnetwork broadcast address just do an AND with your IP and Subnet Mask.
Usually this is the basics of the computer networking(this teaches at the first course of computer networking).