Subnet Masks

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just seen it thanks! lets see....

 

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You are getting confused by the odd way MS looks at CIDR and classful subnetting. When you do CIDR, classless, subnetting you have to forget about the Class A (255.0.0.0), Class B (255.255.0.0), and Class C (255.255.255.0) as reference points for finding out what a network address or subnet mask is. They exist as reference points only in Classful networking.

Thus just because an IP address in a CIDR network scheme is 174.43.114.224 it doesn't mean it has a default subnet mask of 255.255.0.0. It's impossible to tell what the subnet mask is because of this. Who knows? Someone could have broken down a larger subnet to where the IP address and subnet mask could be 174.43.114.224/28.

This was done so that it is possible to create smaller blocks of IP addresses for those who can't afford, and don't need, a full class A, B, or C address space. In the old Classful system you had to buy or lease a full Class C address block (255 IP addresses) even if all you needed for your business was 3 IP addresses. CIDR makes this possible by allowing the breakup of the networking blocks into much smaller pieces. And because this is possible it's impossible to know what your network address and subnet mask are with just an IP address.

 

Now thats an explanation! (see above)

 

Guys... this is all very clear now but if someone could explain to me exactly how im going to work something like this out?

From binary to the answer.....

EDIT: sorted out now!

i think its time i went back to me books!

 

In this case the keys are 1) It's Microsoft - so they cling onto classes even in a classless world, and b) the hint that they wanted a CIDR representation.

So by 1) you use the classful rules, and only at the end show the CIDR equivalent.

Thats how I read the wording anyway.

Note that A+ and even N+ (which is a newer exam) still continue to major on classes. I suspect this is part-driven by the reluctance of M$ to acknowledge CIDR properly.

Harry.

 

Oh and Thanks again to all of you... i owe you one big beer each after this!



Cheers!