An Internet Service Provider (ISP) has the following chunk of CIDR-based IP addresses available with it: The ISP wants to give half of this chunk of addresses to Organization A, and a quarter to Organization B, while retaining the remaining with itself. Which of the following is a valid allocation of addresses to A and B?

Options are :

  1. and
  2. and
  3. and
  4. and

My attempt:

Given address is : = 11110101.11111000.1000 0000.00000000

so, we have last 12 bits for hosts . Now if we give half hosts to Organization A , (now we have only last 11 bits for hosts).possible address for Organization A .i.e., = 11110101.11111000.10000 000.00000000

and also , = 11110101.11111000.10001 000.00000000

and a quarter to Organization B ,i.e. , = 11110101.11111000.100000 00.00000000

Now , we have remaining last 10 bits for hosts.

Hence , both option (1) and (2) are matches.

Why option (2) is wrong ?

Can you explain it in a formal way, please?

This question from competitive exam GATE (see-Q-no.-34) and answer key is given by GATE is option (1) (see-set-A-Q-no.-34) .

  • It seems like you're missing something obvious about the point of having network addresses in the first place: If the ISP assigns to Organization A and to Organization B, and the ISP receives a packet addressed to, who should they send it to? =) – rakslice Oct 2 '15 at 8:05
  • @rakslice: AFAIK the most specific route would be chosen, no? – user1686 Oct 2 '15 at 8:07
  • overlapping, rt ? – ً ً Oct 2 '15 at 8:07

Option 2 is wrong since the first mask includes all addresses from the second one; it is a superset. (Notice that the bit prefixes match.) This means that some of the addresses will be given to both organizations. You can't really tell Org A to "use this address range – except this address, and that one, and that…"

Specifically, starts at and ends at
Meanwhile, starts at and ends at
So you can see that Org B would be using addresses from Org A's space.

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