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1750 Views 5 Replies Latest reply: Jun 18, 2012 12:55 AM by Sey RSS

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172.31.0.0/24 is subnetted ,10 subnets  what does this mean?

Jun 17, 2012 12:21 PM

swaminathan 58 posts since
Dec 28, 2010

Hi

 

In ICND2 I have found this theory in one example for show ip route.....Gateway of last resort is not set, 172.31.0.0 /24 is subnetted , 10 subnets and it is giving the subnets as 1. 172.31.130.0 .172.31.140.0,..........

 

What is my confusion is 172.31.0.0 /24 is subnetted means the subnetting should start after 24 bits .? the subnets are in 3rd octet ..

 

May be I am wrong in my interpretation ! Please anybody kindly explain this

 

Thanks & Regards

 

S.Swaminathan

  • Sey 1,388 posts since
    May 4, 2010

    I agree that this is confusing. I'll try to explain that.

     

    The classful network is 172.31.0.0/16, so if you had a subnet with any other prefix length (let it be 172.31.250.32/27) in the routing table, you would see something like

     

    172.31.0.0/16 is variably subnetted, 11 subnets, 2 masks

    172.31.130.0/24 ...

    172.31.140.0/24 ...

    ...

    172.31.250.32/27 ...

     

    However, when there is only a single prefix length within a classful network like in your example, the output of show ip route looks a bit different. The header of the classful network specifies the prefix length, and the following network addresses don't have the prefix length:

     

    172.31.0.0/24 is subnetted, 10 subnets

    172.31.130.0 ...

    172.31.140.0 ...

    ...

     

    As you can see it's not about subnetting but just a notation.

  • Sey 1,388 posts since
    May 4, 2010

    It's 255.255.255.0 because of /24 in the header "172.31.0.0/24 is subnetted"

  • Sey 1,388 posts since
    May 4, 2010
    That means 172.31.0.0 /24 is not a classful network.It should be a classless

    It's not a classful network but a prefix within a classful network. But in the header it's not about that. It means that the following subnets will all have the same prefix length, and that is /24.

    and you have mentioned in your previous post "

    However, when there is only a single prefix length within a classful network like in your example, the output of show ip route looks a bit different. The header of the classful network specifies the prefix length, and the following network addresses don't have the prefix length:

     

    172.31.0.0/24 is subnetted, 10 subnets

    172.31.130.0 ...

    172.31.140.0 ..."

     

    You are meaning that it comes under classful or classless network?

    I mean classful. 172.31.0.0/24 in the header means that there is no other prefix length within the classful 172.31.0.0/16 network. Otherwise it would read "172.31.0.0/16 is variably subnetted, XX subnets, YY masks".

     

    I repeat, this has nothing to do with subnetting. It's just a notation that Cisco chose to use.

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