Need some help with subnetting

gbdavidxgbdavidx Member Posts: 840
I am practicing subnetting from so I can get better at it and fast at it, I understand the concepts and the basics

However on one question it wants to know the last usable host, ip address was with a /21 subnet mask

I have the correct answer, but am confused on how to get it, appreciate the help!!


  • pamccabepamccabe Member Posts: 315
    Hi gbdavidx, I'm studying for the ICND1 exam and am trying to get better with subnetting. Hopefully I can help you out. First, a /21 prefix means that the first 21 bits are in an 'on' state. This means the first 2 octets are full, leaving us 5 left over. 5 bits of the first octet is a 248 mask. Using the magic number method, we subtract 256 from 248 to get 8 as a magic number. would be the subnet ID, the last usable host would be 143.254 if I am not mistaken. would be the next subnet and would be the broadcast. Someone correct me if I am wrong.
  • gbdavidxgbdavidx Member Posts: 840
    how do you get from to
  • pamccabepamccabe Member Posts: 315
    Well the subnets would increment by 8 so you have followed by and so on and so on... the IP range would start at, and go up from there. Once you hit 255 the 3rd octet would increase by one. All the way up to because the next address of would be the broadcast for the subnet. I hope that makes sense.
  • goldenlightgoldenlight Member Posts: 378
    To Keep my self organize while doing sub netting involving host I draw 3 columns

    Naming them Networks, Host , and Broadcast. when I have my Network and Broadcast address, everything in between are my host

    I calculate a few networks and broadcast address and go from there.
    The Only way to do great work is to love what you do. If you haven't found it keep looking. Don't settle - Steve Jobs
  • Ltat42aLtat42a Member Posts: 587 ■■■□□□□□□□
    You could add a 4th column, name them "Network" (or Subnet), "1st useable address", "last useable address", "Broadcast".

    This will show you the subnets, host ranges & broadcast address.

    The last useable address is always 1 address BEFORE the broadcast.
  • jsb515jsb515 Member Posts: 253 /21 with a mask

    this is how I do it using the / notation.

    Take /21 and subtract it from /24 and that leaves you with 3. Take the 3 and put it to the power of 2, so 2^3=8 this is your count up on subnets.

    so would count up on the third octet since that is where the mask stops.

    we now know our host range. Its going to be - with the last usable address being The broadcast would be

    I would suggest reading this thread because it helped me so much
  • gbdavidxgbdavidx Member Posts: 840
    TY JSB!!! i was curious how you got from 136 to 144, that explains it! I like this method i'll think i'll stick to it!
  • jsb515jsb515 Member Posts: 253
    glad I could help :)
  • TheNewITGuyTheNewITGuy Member Posts: 169 ■■■■□□□□□□
    just another tip.. you can take the 248 mask and subtract from 256 if you're having trouble memorizing the powers of 2 - thats the block size (or how wide the subnet is) covers this very well.
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