what is classless subnetting? is it similar to CIDR? can anyone help me? thanx!

Classless is the same thing as CIDR. More definitive information here http://www.ietf.org/rfc/rfc1519.txt

Thank you very much!

I just realized that RFC 1519 is obsoleted by RFC 4632 as of last year but I think for a primer its good reference.

Well... I wouldn't say that they're technically the same thing. You could have a CIDR number of /8 on a 10.1.1.0 network, and it would still be classful.

Classless subnetting is basically when you have a range of IP addresses for your subnet that falls out of a specific IP class.

Class A: The first number is from 1 to 126, the subnet mask is 255.0.0.0 (CIDR # is 8)
Class B: The first number is from 128 to 191, Subnet mask 255.255.0.0 (CIDR 16)
Class C: The first numebr is from 192 to 223, Subnet mask 255.255.255.0 (CIDR 24)

If you mix and match these, then it's called a classless subnet, because it doesn't call into any specific class. If you follow one specific class on both IP and subnet, then it is classful.