Simple Subnets. Step by Step Worksheet
Here is a worksheet you can use to do simple subnetting. Examples are at
the bottom.
(Note: Old Cisco CCNA classes teach that the first and last subnets
are unusable. Cisco CCNP classes do not teach this, and it is no
longer true for newer routers. In the examples below: If you
see references to "old" routers, then consider the first and last subnets
to be unusable. If not, then you can assume "new" routers and
that all subnets are usable. Some of the examples are done both ways.
)
1. Default Network Address (given): __________________
1a. Number of bits you have to work with:
2. Local Network Requirements (given)
2a. Minimum Number of Subnets needed (if any): ________
2b. Minimum Number of Hosts needed (if any): ________
3. Figure How Many Bits to "borrow" and to "leave".
If you are "given" the number of subnets needed,
then you figure how many bits you need to "borrow" for that many subnets.
(Find the number of bits (N), so that your
required number of subnets is less than or equal to 2**N.
[ 2**N - 2 for "old" routers])
Subtract "N" from your answer in 1a and you have the number of bits for hosts "H".
If you are "given" the number of hosts needed,
then you figure how many bits you need to "leave" for that many hosts.
(Find the number of bits (H), so that your
required number of hosts is less than or equal to 2**H - 2.)
Subtract "H" from your answer in 1a and you have the number of bits for subnets "N".
3a. Record your answers:
number of bits to "borrow" for subnets (N):
number of bits to "leave" for hosts (H):
4. Local Network Design. Calculate
4a. New Maximum Number of Subnets (2**N or 2**N - 2):
4b. New Maximum Number of Hosts (2**H - 2):
4c. Number of IP Addresses for each subnet (2**H):
(N=number of bits for subnets; and H=number of bits for hosts):
Stop here and take a breath!
5. Write out the key addresses for each subnet
Subnet# NetworkAddress IP Range BroadcastAddress
0
1
2
3
...
Fill in the NeworkAddresses first.
Subnet "0" has the same network address as the default network address (given in step 1).
For Subnet "1" just add the answer in 4c to the value for subnet "0".
For Subnet "2" add the answer in 4c to the value for subnet "1".
etc.
The BroadcastAddress is one less than NetworkAddress of the next subnet.
The Subnet's IPRange are the IP's between its Network and Broadcast Address.
6. Calculate the new network mask.
6a. For a Class C address, the default network mask is: 255.255.255.0
6b. Write out the 8 bits for subnets and hosts in binary.
Use 1's for the subnet part, and 0's for the host part.
6c. Convert that to decimal. If you do not have a calculator
then you can do the conversion by hand.
First, make a table with 8 columns like below.
decimal value: 128| 64| 32| 16| 8| 4| 2| 1|
Then put your answer from 6b under it, one bit under each column.
Then for each column where there is a '1' add in the decimal value.
The sum is the new value for the last byte of your network mask.
6c. New Network Mask _______ . ________ . ________ . ________