Class B subnettting

hassantalal785

Here is a question :

Given a class B network of 155.44.200.0, what mask could be used to provide 15 networks each supporting 120 users :
A 255.255.252.0
B 255.255.248.0
C 255.255.240.0
D 255.255.255.128
E 255.255.255.240
The correct answer is "C".
But i think option "A","B","C"and "D" are correct

Because if i want 15 networks so i will use 4 network bits and for 120 users 7 host bits .So if i use 255.255.252.0 (Option A) i will have Network Bits = 6 and Host bits =10 .If i use 255.255.248.0 i have Network Bits = 5 and Host bits =11 .For 255.255.240.0 i have Network Bits = 4 and Host bits =12 and for 255.255.255.128 i haveNetwork Bits = 9 and Host bits =7.

I know i am missing something but cant figure out what.Kindly explain this in detail.

NetworkVeteran

hassantalal785 wrote: »

Here is a question :

Given a class B network of 155.44.200.0, what mask could be used to provide 15 networks each supporting 120 users :

2^3 = 8, 2^4 = 16. You need 4+ subnet bits to support 15+ subnets.
2^6-2 = 62, 2^7-2 = 126. You need 7+ host bits to support 120+ users.
A class B network mask eats up 16 bits.

a) A - 255.255.252.0 - A /22 w/ 16 net bits, 6 subnet bits, and 10 host bits. Pass.
b) B - 255.255.248.0 - A /21 w/ 16 net bits, 5 subnet bits, and 11 host bits. Pass.
c) C - 255.255.240.0 - A /20 w/ 16 net bits, 4 subnet bits, and 12 host bits. Pass.
d) D - 255.255.255.128 - A /25 w/ 16 net bits, 9 subnet bits, and 7 host bits. Pass.
e) E - 255.255.255.240 - A /28 w/ 16 net bits, 12 subnet bits, and 4 host bits. Fail.

The correct answer is "C".
But i think option "A","B","C"and "D" are correct

If it allows you to select multiple answers, they're simply wrong. None of these answers provide exactly 15 subnets, and A-D all meet the requirements. If they asked for a single answer and they asked for 16 subnets, C would be the best answer.

hassantalal785

The given Class B address is 155.44.200.0 .Wouldn't 200 in the third octet play in role in determining the mask ?

NetworkVeteran

The given Class B address is 155.44.200.0 .Wouldn't 200 in the third octet play in role in determining the mask ?

I would just consider it another error. 155.44.0.0/16 is a class B network. 155.44.200.0 is not. It's just one of the IP addresses belonging to 155.44.0.0/16. I've taken the CCNA exam multiple times and I don't recall ever scoring less than 100% on subnetting. The subnetting questions on the exam tend to be precisely worded and the answers unambiguous.

hassantalal785

NetworkVeteran wrote: »

155.44.0.0/8 is a class B network. 155.44.200.0 is not.

Can a Class B address have a /8 mask ?

NetworkVeteran

hassantalal785 wrote: »

Can a Class B address have a /8 mask ?

Typo. They all have a /16 network mask. A full list of class B networks is found here--

http://www.aturtschi.com/whois/networks.html

As you'll notice, 155.44.0.0 is listed. 155.44.200.0 is not.



155.044.0.0

Houghton Mifflin Company

NET-HMCO-NET

<removed>

Boston, MA 02108

chopsticks

I think this question is incorrectly asked because there are more than one answer that fit the requirement.