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.
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