subnetting confusion..

sleemie

looking at this question got me confused. I thought that /27 means that the mask is using 27 bits, therefore making it automatically a class C address using the first 24 bits by default and then stealing an additional three bits from the last octet making the mask 255.255.255.224...how can any of the other addresses possibly be valid if they're not class c addresses?

http://www.techexams.net/forums/viewtopic.php?t=13062

Also, I came across a little subnetting practice test where through all kinds of subnetting questions at you, but I can't find it now...not sure if it was on this site or not.

okay...just thought about this some more and I THINK I've got it partially figured out, but any additional info would be helpful. I did see something at one point where you could cross octets with subnetting, so you could have for example a class A or B address with a mask of 255.255.255.224 or 240, etc.

Any more elaboration would be helpful, also verification that I'm correct, i'm just kind of guessing here based on my just having begun studying this stuff with no books yet, so my resources are limited at this point.

eurotrash

you can have any mask with any address. well not any of course there are the exceptions and exclusions, but like 10.0.0.10/27 is a valid address as is 192.168.0.10/4.
know the classes but don't feel bound by them.

Humper

_omni_ wrote:

know the classes but don't feel bound by them.

Thats soooo true!

gravyjoe

10.0.0.10/27: This just means that a whole lot of subnets are being used. The bigger the bits, the more subnets are used, and the less hosts are being used.

192.168.0.10/4: This is a great example of a config that is not using many subnets, but has many hosts for each subnet.