How to calculate the IP
Range in Network Address
If someone gives you an IP address of 97.156.253.28
and a subnet mask of 255.255.255.248, how do you determine the network address
and the broadcast address, in other words the boundaries, of your network? The
following sections can be learn to get IP Range in network address.
Manual Calculation
1. Subtract the last octet of the subnet
mask from 256 to give the number of IP addresses in the subnet. (256 - 248) = 8
2. Divide the last octet of the IP address
by the result of step 1; don't bother with the remainder (for example 28 / 8 =
3). This gives you the theoretical number of subnets of the same size that are
below this IP address.
3. Multiply this result by the result of
step 1 to get the network address (8 x 3 = 24). Think of it as the third subnet
with 8 addresses in it. The network address is therefore 97.156.253.24
4. The broadcast address is the result of
step 3 plus the result of step 1 minus 1. (24 + 8 -1 = 31). Think of it as the
broadcast address being the network address plus the number of IP addresses in
the subnet minus 1". The broadcast address is 97.156.253.31
Let's do this for 192.168.4.56 with a mask of
255.255.255.224:
1. 256 - 224 = 32
2. 56/32 = 1
3. 32 x 1 = 32. Therefore the network base
address is 192.168.4.32
4. 32 + 32 - 1 = 63. Therefore the
broadcast address is 192.168.4.63
Let's do this for 10.0.0.75 with a mask of
255.255.255.240
1. 256 - 240 = 16
2. 75/16 = 4
3. 16 x 4 = 64. Therefore the network base
address is 10.0.0.64
4. 64 + 16 -1 = 79. Therefore the
broadcast address is 10.0.0.79
Note: As a rule of thumb, the last octet of your network base
address must be divisible by the "256 minus the last octet of your subnet
mask" and leave no remainder. If you are sub-netting a large chunk of IP
addresses it's always a good idea to lay it out on a spreadsheet to make sure
there are no overlapping subnets. Once again, this calculation exercise only
works with subnet masks that start with "255.255.255".
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