# subnet id, why option (2) is wrong? [closed]

An Internet Service Provider (ISP) has the following chunk of CIDR-based IP addresses available with it: 245.248.128.0/20. 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. 245.248.136.0/21 and 245.248.128.0/22
2. 245.248.128.0/21 and 245.248.128.0/22
3. 245.248.132.0/22 and 245.248.132.0/21
4. 245.248.136.0/24 and 245.248.132.0/21

My attempt:

``````245.248.128.0/20 = 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.,

``````245.248.128.0/21 = 11110101.11111000.10000 000.00000000
``````

and also ,

``````245.248.136.0/21 = 11110101.11111000.10001 000.00000000
``````

and a quarter to Organization B ,i.e. ,

``````245.248.128.0/22 = 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 245.248.128.0/21 to Organization A and 245.248.128.0/22 to Organization B, and the ISP receives a packet addressed to 245.248.128.1, 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