Quick subnetting question...please help

chickenlicken09chickenlicken09 Member Posts: 537 ■■■■□□□□□□
in CCNA & CCENT
Could someone please explain to me why 172.26.224.1 is the first valid host address on the 172.26.235.62 255.255.240.0 network ? Specifically the 224.1 part. I know this should be an easy one I'm just stuck.

When I worked it out 224 was working out as the subnet address therefore not valid, wrong?
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  • shellee1983shellee1983 Member Posts: 71 ■■□□□□□□□□
    eddo1 wrote: »
    Could someone please explain to me why 172.26.224.1 is the first valid host address on the 172.26.235.62 255.255.240.0 network ? Specifically the 224.1 part. I know this should be an easy one I'm just stuck.


    this is due to the fact that this is a classless network, 255.255.240.0, and you have to deduct the magic number here.

    240 is the magic number i.e. 256 - 240 = 16...16, 32, 64, ... 208, 224, 240 (multiples of 16) 240 is more than 235 thus making the only logical valid host as 172.26.224.1 (as you cannot use the subnet zero).
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  • cisco_nerdcisco_nerd Member Posts: 198
    eddo1 wrote: »
    Could someone please explain to me why 172.26.224.1 is the first valid host address on the 172.26.235.62 255.255.240.0 network ? Specifically the 224.1 part. I know this should be an easy one I'm just stuck.

    When I worked it out 224 was working out as the subnet address therefore not valid, wrong?

    An easy way to work this out is get your address, convert the interesting octet of the ip address and subnet mask into binary and do a logic AND on them. This will tell you the network that address belongs to.

    255.255.11110000.00000000 (mask)
    172. 26.11101011.00111110 (subnet)
    =
    172.26.11100000.00000000
    =
    172.26.224.0 (network)

    OR you can work out the increment and go from there.

    A mask of 255.255.240.0 or /12 means you would have an increment of 16 in the 3rd octet. So if you count up you will get:

    172.26.0.0
    172.26.16.0
    172.26.32.0
    ...
    172.26.208.0
    172.26.224.0
    172.26.240.0

    This means that your address of 172.26.235.62 is in the 172.25.224.0 network. Therefore the first valid address is 172.26.224.1.

    Hope that makes sense.
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  • cisco_nerdcisco_nerd Member Posts: 198
    shellee1983 wrote: »
    this is due to the fact that this is a classless network, 255.255.240.0, and you have to deduct the magic number here.

    240 is the magic number i.e. 256 - 240 = 16...16, 32, 64, ... 208, 224, 240 (multiples of 16) 240 is more than 235 thus making the only logical valid host as 172.26.224.1 (as you cannot use the subnet zero).

    just to clarify your point...

    You mentioned here that you cann't use the subnet zero command. With VLSM, as used here, it is implied and required that the zero subnet is supported... without specifically defining the router to reject the zero subnet, or using a classful routing protocol, the the zero subnet (and broadcast subnet for that matter) are used.

    Its not stated that the no ip subnet-zero command is used, or RIPv1 is the protocol.. so i wouldn't say it can't be used.
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  • shellee1983shellee1983 Member Posts: 71 ■■□□□□□□□□
    cisco_nerd wrote: »
    just to clarify your point...

    You mentioned here that you cann't use the subnet zero command. With VLSM, as used here, it is implied and required that the zero subnet is supported... without specifically defining the router to reject the zero subnet, or using a classful routing protocol, the the zero subnet (and broadcast subnet for that matter) are used.

    Its not stated that the no ip subnet-zero command is used, or RIPv1 is the protocol.. so i wouldn't say it can't be used.

    Yes, I was not talking about the command I was just talking about the 0 address as it is reserved, however some practice exams have thrown me for a loop on that saying you cannot count that 0. I honestly cannot remember the terminology for it. icon_redface.gif but I'm glad someone knew what I was talking about.
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  • cisco_nerdcisco_nerd Member Posts: 198
    shellee1983 wrote: »
    Yes, I was not talking about the command I was just talking about the 0 address as it is reserved, however some practice exams have thrown me for a loop on that saying you cannot count that 0. I honestly cannot remember the terminology for it. icon_redface.gif but I'm glad someone knew what I was talking about.

    If you are referring to the network ID (172.26.224.0) in this case, then yes it is reserved. It is reserved as it is the Network address for that subnet. just as 172.26.239.255 woud be reserved as the broadcast address for that subnet. With useable hosts in the range 172.26.224.1 -> 172.26.239.254.
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  • shellee1983shellee1983 Member Posts: 71 ■■□□□□□□□□
    cisco_nerd wrote: »
    If you are referring to the network ID (172.26.224.0) in this case, then yes it is reserved. It is reserved as it is the Network address for that subnet. just as 172.26.239.255 woud be reserved as the broadcast address for that subnet. With useable hosts in the range 172.26.224.1 -> 172.26.239.254.


    Yes! That is precisely what I meant. I knew broadcast address but my brain is so fried right now I couldn't, for the life of me, think of the term (network ID), thanks!
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