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Yes, this is a school question, and yes I already know the correct possible answers (supposedly), so I'm not going to bother posting them. Instead, I just want to understand the meaning of this question. I know what class B addresses are, and what valid class B subnet addresses are. I guess one way to define my question is to ask why the answer "255.255.0.0" is wrong, necessarily? That seems to defy my understanding, or else some part of my brain is just not registering the question correctly.

Thanks in advance.

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You could bother posting the possible answers, always interesting ;) It will also give us a better understanding of the situation, because you give only one answer, and it's hard to know how is it, compared to others. (If, like I understand from what you said, this is a multi-choice question) –  Gnoupi Mar 24 '10 at 15:06
    
That one answer seems to be the pivot point of possibilities, though. I didn't think any of the others were really needed. –  Daddy Warbox Mar 24 '10 at 15:20
    
You're right, I simply had issues understanding exactly the question then, and wasn't sure why 255.255.0.0, so I was curious to see other suggestions. –  Gnoupi Mar 24 '10 at 15:27

1 Answer 1

up vote 3 down vote accepted

I think the key part of the question is that this network is subnetted.

(Quick note, even if you probably know it: class B addresses use the first 16 bits for the network id, and the remaining 16 bits are available for machines).

The mask 255.255.0.0 allows only for one network, all ips from the Class B. If you want to have subnetworks, at least one bit from the machine part of the ip will have to be used for it. As such, you can't use 255.255.0.0, because it doesn't allow you to separate subnets.

Example:

  • 172.16.0.1 to 172.16.128.254 -> subnet A
  • 172.16.129.1 to 172.16.255.254 -> subnet B

Even for only 2 subnets, you will need a mask like 255.255.128.0. (255.255.10000000.00000000)

255.255.0.0 won't allow you to make a difference in subnet.

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So an address isn't subnetted if its only mask is the class itself? So a requirement of a subnet is that it doesn't compass the whole range of class addresses? What do you call an address that isn't part of a subnet, then? –  Daddy Warbox Mar 24 '10 at 15:50
2  
A class-B is a subnet itself. A subnetted class-B implies that the class-B is divided into a number of subnets itself, each of which are smaller than the class-B. –  David Mackintosh Mar 24 '10 at 16:02
    
@David - thanks for clarification. –  Gnoupi Mar 24 '10 at 16:25

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