Contributed By: Cliff Hobbs [MVP SMS]
Do not fear Hobbs is here with an easy to remember table that will allow you to answer ANY subnetting question anyone cares to throw at you ...

If like me you think subnetting is some kind of magical science then fear no more.  The table in this article is the answer to all of your prayers and will allow you to answer any subnetting question such as:

'You have w IP address and x number of hosts - what subnet mask do you need to use?'
'You have y network ID that needs z subnets - what subnet mask do you need to use?'
You can even use this table to answer supernetting questions.

Until I was taught this table by Nigel Amos, I really struggled with subnetting-related questions.  I knew that there had to be a clever way of working this out as you're only using binary numbers, but I don't have the sort of brain to work it out.  If you learn this table which is really easy then I guarantee you'll never wince at another subnetting question.
 

Drawing up the Table

Right let's show you how to draw up the table.

1. Draw a table of 11 columns by 14 rows.  The first two rows will be the header rows.
    
2. In the first column, write 'Ticks' in the first row and 'No Ticks' in the second row.
    
3. Label the second column as 'Temp'.
    
4. Label the first row of the third column as 'Subnets'.
    
5. Label the second row of the third column as 'Hosts'.
    
6. Label the first row of the last eight columns as '128' '192' '224' '240' '248' '252' '254' '255' respectively.
    
7. Label the second row of the last eight columns as '128' '64' '32' '16' '8' '4' '2' '1' respectively.
    
8. In the third row of the first column, write the numbers 1 - 12 in separate rows.
    
9. In the third row of the second column write '2', then double this number in the remaining rows (for example '8', '16', '32' etc. up to '4096').
    
10. In the third row of the third column take 2 away from the corresponding entry in the 'Temp' column (so you start with '0' '2' '6' '14' etc. up to '4094').
     
11. Now cross out the 'Temp' column.

This is what you should end up with.

Using the table

Probably the best way to teach you how to use the table is to give you worked examples.

You have a Network ID of 160.100.0.0 and you need 17 subnets.  What subnet mask would you use?

Look at the third column first.  We're looking for the number of subnets which corresponds to the first row - look across to the first row of the first column which is labelled 'Ticks'.  Now look down the third column till you find a number greater than or equal to 17.  In this case it is '30'.  Read back across to the table to the first row which reads '5'.  Now put 5 ticks starting from the left of the last eight rows.  Go up to the top row which reads '248'.  So your subnet mask is 255.255.248.0.

Let's try another one.

You have a Network ID of 10.0.0.0 and you need 75 subnets.  What subnet mask would you use?

Look at the third column first.  Again we're looking for the number of subnets which corresponds to the first row - look across to the first row of the first column which is labelled 'Ticks'.  So now look down the third column till you find a number greater than or equal to 75.  In this case it is '126'.  Read back across to the table to the first row which reads '7'.  Now put 7 ticks starting from the left of the last eight rows.  Go up to the top row which reads '254'.  So your subnet mask is 255.254.0.0.
Let's try a hosts question.

You have a Network ID of 198.10.15.0 and you need 35 hosts per subnet.  What subnet mask would you use?

Again look at the third column first.  This time we're looking for the number of hosts which corresponds to the second row - look across to the second row of the first column which is labelled 'No Ticks'.  Again, look down the third column till you find a number greater than or equal to 35.  In this case it is '62'.  Read back across to the table to the first row which reads '6'.  Now here's the difference.  We're talking 'No Ticks' this time as we're looking for the number of hosts.  So starting from the RIGHT hand column of the last eight rows count 6 columns back.  Put a tick in the next column to the left.  Now go up to the top row which reads '192'.  So your subnet mask is 255.255.255.192.
 

Are two hosts on the same network?

How many times have you seen questions where two hosts can't communicate and you need to work out whether they're on the same subnet or not?  Well you could:

1. Panic
2. Write everything out in binary and do some fancy maths
3. Fish
4. Bernard
5. Use the table to help

Well as with most questions there are two obvious wrong answers (c. and d. in case you're struggling).  You could go for a. (if you're human) or b. if you're good with numbers or like mental pain.

The answer in this case is e. which will hopefully not make a. your automatic choice for future questions.  Let's look at an example.

You've got two hosts:
132.25.5.18
132.25.8.44

Both hosts are using a subnet mask of 255.255.248.0.  Are they on the same subnet?

Using the table we can work this out.  Focus on the top two rows of the last eight columns.  Look at the last octet in the mask which has a value, which in this case is '248'.  Find '248' on the top row and look at the number immediately below it which in this case is '8'.

This means that each new subnet starts on a multiple of 8 in the third octet.  So the first valid subnet address range would be:
132.25.0.0 to 132.25.7.254

The second would be
132.25.8.0 to 132.25.15.254
etc.

So in our example, we can see that the clients are in fact on different IP subnets.

Supernetting
Well I promised you this table did everything and supernetting is no exception.  Let's look at an example.

Your ISP has given you four IP addresses as follows:
200.100.104.0
200.100.105.0
200.100.106.0
200.100.107.0

Now using a default subnet mask of 255.255.255.0 these IP addresses aren't on the same subnet so we need to modify the subnet mask.  But to what?  Don't worry.  You've got four Network IDs in this example so starting from the left put 4 ticks in the table.  Go up to the top row which reads '240' so your subnet mask would be 255.255.240.0 - note that it's NOT 255.255.255.240 as we're dealing with supernetting here.

The other thing to watch is if you get asked a question with an odd number of IP addresses.  For example if you're given 5 IP addresses work out your answer based on the next even number (6 in this case or '252').

In Summary
I appreciate that trying to explain something like this through an article is very difficult.  If you don't understand anything please contact me and I'll try to help you.

Useful subnetting links
I'd like to thank various people (notably Nigel Amos and Dan DiNicolo), who have allowed me to 'see the light' and sent me useful subnetting links in my quest to try to understand how this all works.

If you want some free practise exams to try your hand at subnetting go to the Windows 2000 Trainers site:

http://www.2000trainers.com/exams/subnetting/

Still don't get it?  This URL says it all:

http://www.learntosubnet.com

If you know of any other useful links let me know and I'll add them in.