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5682 Views 4 Replies Latest reply: May 15, 2009 7:24 AM by Jared RSS

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Configure mask /29 and /30 on a router

May 14, 2009 8:09 PM

sulley 4 posts since
Jan 28, 2009

Hi all

I spent an hour playing with the CCNA rack at mindtech.com and wasnt able to configure interfaces the way I wanted them.

The routers are running IOS 12.2

My question is : Can you configure a router to have 192.168.1.5 /30 for its Serial0 and 192.168.1.9 /29 for its Ethernet0?

I was not able to do that last night even after making sure ip classless had been enabled. What's wrong?

  • Jared 5,498 posts since
    Jul 27, 2008
    Currently Being Moderated
    1. May 14, 2009 8:45 PM (in response to sulley)
    Re: Configure mask /29 and /30 on a router

    ok, so if you assign an interface an IP of 192.168.1.5 /30, I would assume that .4 is your network, .5 and .6 are your hosts and .7 is your broadcast.  Then a new network of 192.168.1.8/29 where .8 is the network address, .9 is a host, which happens to be the interface.  .10-.14 are also hosts, which would give you a total of 6 hosts and .15 would be the broadcast.

     

    That seems like it should work... if the router knows it is allowed to use the zero subnet, which would be 192.168.1.0/30, which would have .0 as the network, .1 and .2 as hosts and .3 as the broadcast.  Otherwise, it may think there is an overlap and not allow your addressing.

     

    was ip subnet-zero also configured on your device?

  • amine00 133 posts since
    Dec 17, 2008
    Currently Being Moderated
    3. May 15, 2009 4:16 AM (in response to sulley)
    Re: Configure mask /29 and /30 on a router

    Hi Sulley,

    You might be confused about the purpose of the mask, because you thought that it would be possible to give the same IP adress to multiple interfaces but with different masks.

    Actually it's not possible, because the mask in itself is not an identifier for the interface, it's the IP address that identifies uniquely each interface. The mask main purpose is to identify which part on the IP address is the network part, and which is the host part. This lets the interface know which subnet it belongs to.

     

    Regarding your initial question, I agree with Jared about the subnet ranges, but I don't see why the "ip sunet-zero" would have an impact. I don't think so, because your subnets are:

     

    192.168.1.4 /30 (which 192.168.1.5 /30 belongs to).

    192.168.1.8 /29  (which 192.168.1.9 /29 belongs to).

     

    I don't know what's wrong with your config,

  • Jared 5,498 posts since
    Jul 27, 2008
    Currently Being Moderated
    4. May 15, 2009 7:24 AM (in response to sulley)
    Re: Configure mask /29 and /30 on a router

    Sully,

     

    I see that you have marked your question as answered.  Hopefully it was just a fat finger in your config or the fact that you tried to have the same IP on two interfaces.

     

    I just set up your original explained config with 192.168.1.5/30 on one interface and 192.168.1.9/29 on another interface in my test lab and had no issues.

     

     

    What did you find to resolve the problem?

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