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Page last modified:  05/27/03 03:06 PM

Subnetting Network Addresses

1.  Determine the number of networks that you will need.

Figure out how many network IDs that you will need.  You will need a subnet for each physical network.  You can organize your company by department or geography.  Each router interface will require its own subnet.    Once you determine your number add one, then convert that number to binary.  For example if you are planning for 5 networks, use the number 6 (5+1).  Convert the number six to binary or 110.

2.  Determine your subnet mask.

Take the number of bits that you used for binary 6 and covert them to high order in an 8 bit octet.  We've used three bits for the number six.  When placed in high order, we obtain this number: 11100000. Calculate your subnet mask by converting this number to decimal. 11100000 equals 224.  This will be the last number in the subnet mask.  

3.  Do you have enough host IDs?

Take a minute and double-check if you have enough IP addresses available for hosts.  Don't forget that every computer needs a host ID.  So does each router interface and network printer.  You can calculate this number by using the following formula:  2 ^N - 2.  N represents the number of bits left over.  A Class B subnet mask of 255.255.224.0, uses 19 bits.  There are 32 bits in an IP address.  32-19 equal 13.  In this example the formula will read 2 ¹³ - 2 or 8190.  This is how many host IDs are available.

4.  Calculate your delta.

The delta is simply the numeric value of the right most high order bit in the subnet mask.  224 is expressed in binary as 11100000.  Looking at the right most 1, determine the binary value.

5.  Calculate your network IDs and IP address ranges.

The delta becomes the first network ID.  Suppose we purchased a network ID of 131.107.0.0.  Using the custom subnet mask of 255.255.224.0 our first valid network ID is 131.107.32.0.  Add the delta to the third octet to obtain the second network ID of 131.107.64.0.  The starting IP address is the network ID plus 1.  The ending IP address is one less than the next network ID.