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3179 Views 5 Replies Latest reply: Apr 9, 2009 4:00 AM by chris0161 RSS

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SLSM: When to use the zero subnet?

Apr 8, 2009 1:48 AM

chris0161 7 posts since
Jan 17, 2009

 

Hello Everyone

 

 

I am studying subnetting for my CCENT exam and I was wondering if anyone could help out. I am uncertain when to use the zero subnet when deciding which subnets from an available range to use. Here is an example question:

 

 

An engineer has to assign the first IP address from subnets 3 and 4 in network 172.168.0.0/19 onto ports Fa0/0 and Fa0/1 of a router. What would be the addresses and masks?

 

 

172.168.0.0

 

 

172.168.32.0

 

 

172.168.64.0

 

 

172.168.96.0

 

 

172.168.128.0

 

 

172.168.168.0

 

 

172.168.192.0

 

 

172.168.224.0

 

 

In Wendell Odom's Cert Guide he says to use the zero subnet unless class-full subnetting protocols are used (RIP 1) or the no IP subnet zero command has been issued. Therefore I would answer this question as:

 

 

Fa0/0: 172.168.64.1 255.255.224.0

 

 

Fa0/1: 172.168.96.1 255.255.224.0

 

 

But is this correct? Should I count the zero subnet as a usable subnet?

 

 

Any help much appreciated. Apologies if this has been discussed before.

 

 

  • Paul Stewart  -  CCIE Security, CCSI 6,952 posts since
    Jul 18, 2008
    Currently Being Moderated
    1. Apr 8, 2009 2:00 AM (in response to chris0161)
    Re: SLSM: When to use the zero subnet?

     

    I think your answer is correct.

     

     

    172.168.0.0

     

     

    172.168.0.0 /19 (255.255.255.224) > First subnet

     

     

    172.168.32.0/19 > Second Subnet

     

     

    172.168.64.0/19 > Third Subnet

     

     

    172.168.96.0/19 > Fourth Subnet

     

     

     

     

     

    So the .1 ip address on the third and fourth subnet would be the answer to the question as posed.

     

     

  • Paul Stewart  -  CCIE Security, CCSI 6,952 posts since
    Jul 18, 2008
    Currently Being Moderated
    3. Apr 8, 2009 2:22 AM (in response to chris0161)
    Re: SLSM: When to use the zero subnet?

    I would call the zero subnet, the first subnet. Sometimes in when counting 0 is the first number.

  • Paul Stewart  -  CCIE Security, CCSI 6,952 posts since
    Jul 18, 2008
    Currently Being Moderated
    4. Apr 8, 2009 9:01 AM (in response to chris0161)
    Re: SLSM: When to use the zero subnet?

    Let me go one step further. Either way the subnets are numbered the same. In other words, the only way you would count them differently is if they said something to the effect of "third and fourth 'useable' subnet" alongside some other reason to make you believe that subnet-zero was not permitted.

     

     

     

     

     

     

     

    So with the wording of the original question it would not matter. Even if it stated "no ip subnet-zero", 172.168.0.0/19 is still the first subnet. It just isn't useable. So you still have the same 3rd and 4th subnet. The only way this would be otherwise would be with the "no ip subnet-zero" in conjunction with the verbiage "useable addresses".

     

     

     

     

     

     

     

     

     

    I would even have to think about if RIPv1 is mentioned. I'm not sure that would trigger the "no ip subnet-zero" behavior. There is nothing with the packet format that would keep it from working 0's and 1's subnet. There may be some trivial issue as to why it would trigger the behavior though.

     

    Message was edited by: Paul Stewart

    Below is a screenshot of RIPv1 Updates on the 0 network. So I don't think there would be any reason that RIPv1 would not work with ip subnet-zero. Of course, the subnets must not be dis contiguous.

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