Class B subnettting

hassantalal785hassantalal785 Posts: 47Member ■■□□□□□□□□
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.

Comments

  • NetworkVeteranNetworkVeteran Posts: 2,338Member ■■■■■■■■□□
    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.
  • hassantalal785hassantalal785 Posts: 47Member ■■□□□□□□□□
    The given Class B address is 155.44.200.0 .Wouldn't 200 in the third octet play in role in determining the mask ?
  • NetworkVeteranNetworkVeteran Posts: 2,338Member ■■■■■■■■□□
    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.
  • hassantalal785hassantalal785 Posts: 47Member ■■□□□□□□□□
    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 ?
  • NetworkVeteranNetworkVeteran Posts: 2,338Member ■■■■■■■■□□
    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. :p



    155.044.0.0

    Houghton Mifflin Company

    NET-HMCO-NET

    <removed>

    Boston, MA 02108



  • chopstickschopsticks Posts: 389Member
    I think this question is incorrectly asked because there are more than one answer that fit the requirement.
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