I am trying to remember the trick I used for the CCNA test that worked really well. You can't bring your tables into the test, but what I did, was create all my tables and tips before I started the tests. This one works well for trying to figure out how many subnets and hosts will be available for each bit you borrow.

You set up your table (could be across or down) with

1) The number of bits across the top (don't worry if it isn't valid - the point is to quickly create the table under stress and the test is stressful).

2) The next row will be your number of networks and starts with the number 2 (number of possible patterns per bit - 0 and 1). This should be under the number 1. Keep doubling the number until you get to the end. For example, 2 doubled is 4, 4 doubled is 8, 8 doubled is 16 etc.

3) The last row is your number of hosts. You don't need to calculate it. Just take the number at the end of the previous row and put at the beginning, under the first column, which will have 1 and 2 in the first 2 rows (in a class C network with 8 bits, this will be 256, if class B it will be 65536). Then take the next to the last and put it under the 2nd row (in class C this will be 128). What you are doing is just reversing the numbers. Continue until you fill the row. When you are done, you should find 2 in the last column.

It should look like the following for a class C network:

[code:1]

# bits 1 2 3 4 5 6 7 8

# Networks 2 4 8 16 32 64 128 256

# Hosts 256 128 64 32 16 8 4 2

[/code:1]

These are the number of POSSIBLE networks and hosts (not usable). For USABLE, subtract 2 from each number.

What you do is figure out how many bits you want to borrow and the number of POSSIBLE networks will be directly below. The number of POSSIBLE hosts will be down and one to the right. For example, in the Class C network above, if you want to borrow 5 bits, you would :

1) Look for the number 5 in the first row and go directly down and in the 2nd row you would find 32. This is your number of POSSIBLE networks. Your USABLE networks will be 32 (32 - 2 = 30).

2) Now, go directly down and to the right and you will find the number 8. This is your number of POSSIBLE hosts. Your number of USABLE hosts is 6 ( 8 - 2 = 6).

Now the reason why you don't have set up valid bits (1, 7 and 8 are not valid), is because you will figure it out when you subtract 2 for to get your number of usable networks. For example, if you tried to pick 1, you would find 2 POSSIBLE Networks, but when you subtract 2 you get 0 networks (obviously, this won't fly). If you pick 7, you will get 2 POSSIBLE hosts, but after subtracting 2 to get the number of USABLE hosts, you will get 0 (again, what would be the point). 8 bits won't work as there is nothing in the hosts row.

This may look a little complicated to do at test time, but believe me it isn't and only takes a couple of minutes to set up. It works for all classes (for class B - you would have 16 columns and for class A - you would have 24 columns). When you first sit down at the table, you just build all three tables, and then you won't have to worry about it during the test. If you try to figure it out each time it comes up - and it will come up a few times during the test - you can easily make a mistake in your calculation (and we don't want that - do we?).