Subnet Question

Discussion in 'Network+' started by Smegal, Apr 12, 2019.

  1. Smegal

    Smegal New Member

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    Have a small network with a single subnet connected to the internet.
    Router is the default gateway is has been assigned
    192.168.12.34/28

    I needed to figure out the following:
    IP Address
    Subnet Mask

    I figured out the Subnet mask of 255.255.255.240 from the CIDR 28 but I can't figure out how they came up with the IP.

    **Answer is 192.168.12.46
    Their explanation is with a 28bit mask, the router is on subnet 192.168.12.32, and valid addresses are 192.168.12.33 to 192.168.12.46. Can't use 192.168.12.32 because its the subnet address. Can't use 192.168.12.47 because its the broadcast address.
     
  2. JoeCLewis90

    JoeCLewis90 New Member

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    Based on the answer they are saying is correct, I assume they are asking for the last usable host IP address. This is how I would answer.

    Break 192.168.12.34 into binary, if you would like, but since the last octect is the one with 4 borrowed bits...you could just copy the first 3 octets down, as those will not change in this scenario:

    192.168.12.(__)


    The last octet of the IP (34) converts to 00100010 (Notation in the "32" and "2" places give you 34). You line this up with the binary of the last subnet mask octet:

    Subnet Mask Binary (240) = 11110000
    Last Octet Binary (34) = 00100010

    In this case the last 4 bits of the last octect of the IP are the only numbers that you would manipulate to get the network and broadcast addy. Use the binary of the last subnet mask octet to tell you where to start manipulating the binary of the IP's last octect....
    (where the 0's begin in the subnet mask)

    Zero out the last 4 bits of the binary of the IP (00100000) =32...hence 192.168.12.32 for the Network Address.
    Put 1's in the last 4 bits of the binary of the IP (00101111) =47...hence 192.168.12.47 for the Broadcast Address

    That leaves the 1st and Last usable host address for us to figure....you add one to the Netw. Addy and subtract one from the Broadcast Addy

    00100001 will give you the 1st usable host of = 192.168.12.33
    00101110 will give you the last usable host of =
    192.168.12.46

    I realize that I am reviving an old thread, but they say you know it if you can teach it. So just trying to lend a hand.

    Hope that explanation wasn't too complicated. I tried my best lol
     

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