Skip navigation
Cisco Learning Home > Certifications > IP Networking (CCENT) > Discussions

_Communities

3419 Views 6 Replies Latest reply: Jan 16, 2011 5:09 PM by Martin RSS

Currently Being Moderated

Classless network and subnetting

Jan 15, 2011 11:23 PM

Bouts 3 posts since
Mar 23, 2010

Hi all,

 

This has likely been covered but I can't seem to find the reason behind the answer to this question.

 

You are working on a network that cannot interpret classless addressing. Your network ID is 145.27.0.0. What it the THEORETICAL max. subnets you can create on this network?

 

Answer is apparently: 254

 

WHAT??. This is a classeless network, subnetting it would not work therefore you can't have any subnets. Only networks that can interpret classless networking schemes can use subnetting. Therefore the answer is 0. You can have 1 network but no subnets.

 

BUT, if it the question is asking for THEORETICAL max. number of subnets because you are about to change the network devices to recognise subnetting, the the answer should be 16382 subnets with 2 hosts per network. or 145.27.0.0/30

 

What have I missed in order to not get the answer of 254?

 

I would appreciate any help on the reasoning behind the answer given.

 

Kindest regards,

 

Bouts

  • Brian 2,968 posts since
    Aug 17, 2009
    Currently Being Moderated
    1. Jan 16, 2011 12:45 AM (in response to Bouts)
    Re: Classless network and subnetting

    Aloha Bouts,

     

    Let me see if I can help you out.  Since your network cannot understand "classless" then it means the network is "classful".  Classful pertains to the default Class A, B, and C networks.  You can still subnet these "classful" networks, but the subnets must be the same throughout.

     

    Now taking the Class B network 145.27.0.0 (not sure where you got classless network)  the default mask is 255.255.0.0 or a /16 prefix length.  The number of network bits is 16 and the number of host bits is 16.  To subnet, we borrow bits from the host portion.  If I have 16 bits available what is the largest number of bits I can borrow from 16 to create a new subnet?

     

    Answer: 14

     

    I must leave at least 2 bits for hosts in my new subnet.  This results in a /30 prefix length.  For CCENT or CCNA you will not use the /31 or /32 prefix lengths.  You will learn about these at the CCNP level.

     

    So, coming back to the question, we have the following:

     

    network bits = 16

    subnet bits = 14

    host bits = 2

    # of subnets = (2^14) = 16384

    # of valid hosts per subnet = (2^2) - 2 = 2

     

    Remember, you can not use the network or broadcast addresses, leaving you only 2 valid hosts in each /30 subnet.

     

    Not sure where they got only 254, but I suspect they only borrowed 8 bits from the host portion, taking the Class B address and making it a /24 or 255.255.255.0 subnet.  But this is not the "MAXIMUM" subnets that can be created from a Class B address.  The correct answer should be 16,384 subnets.

     

    HTH

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    3. Jan 16, 2011 12:32 PM (in response to Bouts)
    Re: Classless network and subnetting
    16382 subnets with 2 hosts (2^14 - 2) = 16382,.. a complete stuffup on the editors part in the book ?

     

    NO, in the past, we had to subtract 2 from number of subnetts as well as from number of hosts !

     

    Today, unless you have no ip subnet-zero configured on router or you run RIP 1, you do not have to subtract 2 from number of subnetts. this also means you can use subnet zero.

     

    You always subtract 2 from number of Hosts.

  • Brian 2,968 posts since
    Aug 17, 2009
    Currently Being Moderated
    4. Jan 16, 2011 1:26 PM (in response to Bouts)
    Re: Classless network and subnetting

    Aloha Bouts,

     

    As Martin mention in his post above, you subtract 2 when you have "no ip subnet zero".  I believe since IOS 12.0, "ip subnet zero" is enabled by default allowing you to now use the all "zero" and all "ones" subnets.  So to clarify,

     

    ip subnet zero

    # of subnets = (2^14) = 16384

     

    no ip subnet zero

    # of subnets = (2^14) - 2 = 16382

     

    either way, the 254 the book has is incorrect.  They get 254, by borrowing only 8 bits and assume "no ip subnet zero", so that the # of subnets = (2^8) - 2 = 254.

     

    But this is incorrect, in that you can borrow more than 8 host bits to subnet the Class B address 145.27.0.0.

     

    HTH

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    6. Jan 16, 2011 5:09 PM (in response to Bouts)
    Re: Classless network and subnetting

    get W. Odom 's books. they also have a few misprints (get errata from ciscopress) but none related to subnetting.

    More, you will get video on subnetting by Odom, and practice test ofr your PC

Actions

More Like This

  • Retrieving data ...

Bookmarked By (0)