Subnetting Class A and B addresses

I am having trouble subnetting the class A and B addresses. I can do Class C fine.
For example, how would I subnet these 2 address :
Class A : 10.15.17.22/13 and Class B : 134.72.29.55/20

I am planning to take the CCNA exam and I was told that I have to know subnetting.
Can anyone please guide me or show me how to subnet these class addresses ?

Thanks!!

www.firewall.cx/ip-subnetting-intro.php

for 10.15.17.22/13 (Class A)

the subnet mask for class a is 255.0.0.0 right? well change that to binary which =

11111111.00000000.00000000.00000000

the /13 means there are 13 ones, which means you are borrowing 5 bits.. so mask is written as

1111111.11111000.0000000.00000000

since you are borrowing 5 bits, your total # of USEABLE network will = 2^5 - 2

that leaves you with 19 0's in the address right??

to get total # of USEABLE host = 2^19-2

to check, do 256*256*256
then multiphy host*network, you shoudl get the same answer from 256*256*256

Class B network is the same way.. jsut smaller numbers.. =] hopefully that helped...=]

...i just gave the CCNA..missed it by one question...got a 835...a piece of advicce..before you sit for your exam....know how to subnet like the back of your hand (or any other familiar part...) they have a LOT of questions that are based on subnetting and addressing in general...and the thing is these questions are the ones that take up a lot of time...more often than not...say troubleshooting..the problem is IP addressing....so before you sit for your exam...know THOROUGHLY how to do variable length sunets.....