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Subnet challenge

Andrew NewmanAndrew Newman Member Posts: 23 ■□□□□□□□□□
in CCNA & CCENT
Hi, I don't understand why I can't use subnet 192.168.9.96 /27 in the following example which is from the boson test.I hope you can help me understand it

The internetwork in the diagram uses only private network 192.168.9.0 and RIP2.A new network engineer's manager briefly started configuring the routers, configuring only router R3.The manager says that the engineer should not change R3's IP addresses, but continue the project, choosing subnet numbers and IP addresses for the remaining routers' interfaces.The design goals state that all the remaining LAN-based subnets should use a /27 mask.With all Wan links using a /30 mask.You should assign the smallest available subnet numbers to the LAN-subnets first and then assign the then-smallest-available subnet numbers for any remaining wan subnets.You should then choose interface IP addresses for the routers that are the smallest available IP addresses in each subnet.
The interfaces on R3 are as follows:
FA0/1 192.168.9.33 255.255.255.224
S001 192.168.9.101 255.255.255.252
s010 192.168.9.105 255.255.255.252

Sorry I can't copy and paste the network diagram but there are 3 other LAN subnets to assign subnet numbers to and 2 other routers with
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    mikej412mikej412 Member Posts: 10,086 ■■■■■■■■■■
    Andrew Newman wrote: »
    You should assign the smallest available subnet numbers to the LAN-subnets first and then assign the then-smallest-available subnet numbers for any remaining wan subnets.
    Andrew Newman wrote: »
    The interfaces on R3 are as follows:
    FA0/1 192.168.9.33 255.255.255.224
    S001 192.168.9.101 255.255.255.252
    s010 192.168.9.105 255.255.255.252
    Andrew Newman wrote: »
    Hi, I don't understand why I can't use subnet 192.168.9.96 /27
    The 192.168.9.96/27 subnet would overlap with the existing networks configured on the Serial Ports -- so that subnet is NOT available for that 4th network. But that subnet has already been further subdivided for the R3 serial ports and the other unused subnets in that range will be available.

    Your 4th network would have to use the 192.168.9.129/27 range.

    And when you go to assign that next serial link -- the smallest available subnet would be 192.168.9.96/30

    192.168.9.100/30 is already in use on one of R3's serial ports
    192.168.9.104/30 is already on use on the other R3 serial port.

    The next-available smallest subnet available for another serial interface would then be 192.168.9.108/30

    And if I counted on my fingers correctly -- out of that 192.168.9.96/27 block, there's still a 192.68.9.112/28 piece that is unused. That could be used "as is" or broken up into 2 /29 networks or broken up into 4 /30 networks. Or that remaining piece could be split into 1 /29 and 2 /30 subnet (or 2 /30 and 1 /29 subnets depending on how you split and use it).
    :mike: Cisco Certifications -- Collect the Entire Set!
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    billscott92787billscott92787 Member Posts: 933
    Mike you got to it before me :)

    Smallest available subnets:



    Mike couldn't it use the:

    192.168.9.0 /27 which would be a range of 192.168.9.1 - 192.168.9.30
    Broadcast: 192.168.9.31


    Unless that specific range has a mask that would make it overlap?




    You can't use 192.168.9.96 /27 because this will give you a range of 192.168.9.97 - 192.168.9.126 with a broadcast of 192.168.9.127, this overlaps your current two WAN links:

    S001 192.168.9.101 255.255.255.252


    Subnet: 192.168.9.100
    Range: 192.168.9.101 - 192.168.9.102
    Broadcast: 192.168.9.103



    s010 192.168.9.105 255.255.255.252

    Subnet: 192.168.9.104
    Range: 192.168.9.105 - 192.168.9.106
    Broadcast: 192.168.9.107
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    mikej412mikej412 Member Posts: 10,086 ■■■■■■■■■■
    billscott92787 wrote: »
    Mike couldn't it use the:

    192.168.9.0 /27 which would be a range of 192.168.9.1 - 192.168.9.30
    Broadcast: 192.168.9.31
    They got 3 more /27 networks to "fit in" with the existing addressing -- so that would be where one of those 3 networks could "squeeze" in. Based on the original question I think they already figured out the placement of 2 of those 3 additional networks.
    :mike: Cisco Certifications -- Collect the Entire Set!
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    billscott92787billscott92787 Member Posts: 933
    I thought so. Just figured I would ask :) You're always so helpful thanks alot man.
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    Andrew NewmanAndrew Newman Member Posts: 23 ■□□□□□□□□□
    Thanks guys, you make everything seem so clear.
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    billscott92787billscott92787 Member Posts: 933
    No problem :0 Mike always is good at explaining things and so is everyone else here. This board was one of my best friends in preparation for my CCNA. Actually, to be honest, helping out and posting to people with questions helps me better understand everything and get a better grasp on it.
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    mella060mella060 Member Posts: 198 ■■■□□□□□□□
    Yeh helping people out and responding to posts helps to keep the skills sharp, even if you don't work with networking gear in your day to day job.

    I passed my CCNA back in May and this forum has been great for keeping up to date. Thanks guys !!
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