Subnetting Procedure

This procedure assumes you are beginning with a Class C address block and

must subnet the block to meet the needs of a scenario.

1. Read the scenario. Determine the total number of networks and the

maximum number of hosts in each network. Example: you need to create 6 networks; the largest has 20 hosts.

2. Decide how many bits to borrow from the host portion. Use the formulae.

You can create up to 2

n

networks where n is the number of bits you

borrow. This will provide 2

h

-2 hosts per network where h is the number of

host bits remaining.

Example: Borrow 3 bits from the host portion. This provides 8 networks

(2

3

= 8) and 30 hosts per network (2

5

-2 = 30).

3. Determine the prefix length and subnet mask based on the borrowed bits.

Example: Since we’re borrowing 3 bits, our prefix changes from /24 to /27.

4. Find the “network multiple” by finding the binary value of the last borrowed

bit. To do this, create a binary octet where only the n

th

digit is a 1 where n

is the number of bits you have borrowed from the host portion. The

decimal value of this octet is the network multiple.

Example: Since we have borrowed 3 digits, create the octet 00100000.

The network multiple is 32.

5. Write out network addresses in series using the network multiple to create

the final octet. Start with 0 and add the multiple for each network until you

run out of space.

Example: The network multiple is 32. Therefore the 8 network addresses

will have the final octets: 0, 32, 64, 96, 128, 160, 192, 224.

6. Use the list of network addresses to determine any address in the

scenario, keeping in mind that for any given network, the first address is

the network address, the last address is the broadcast address and all

addresses in between are host addresses.

Example: What are the network, broadcast and host addresses for these

networks? Why can a given host not communicate on the network? What

is a valid host that can be assigned to a host on a given network? etc.

## Comments 0

Log in to post a comment