Can someone help with this classless subnetting of

kartheenkartheen yesMember Posts: 2 ■□□□□□□□□□
Can someone help with this classless subnetting of
Need how many usable hosts, howmany subnetwork comes and under which subnetwor this IP comes.. 
I am specifically looking for the how many subnetwork comes under this prefix and how to calculate it. 


  • Jon_CiscoJon_Cisco Member Posts: 1,770 ■■■■■■■■□□
    edited March 2019
    Can you post your starting point so we can help guide you on the answer. It's easier if we understand where you are getting stuck.

    edit: If you are new and just looking for a starting point you can try an online calculator. If your learning for testing your gonna need to learn to do this in your head fairly quickly.

    I also always recommend this site for practice.

  • clarsonclarson Member Posts: 903 ■■■■□□□□□□
    number of usable hosts:
    it is an ipv4 address which uses 32 bits for addressing.
    /18 means that 18 of those bits are used for the network portion of the address.
    32-18=14  that is the number of bits for the host portion of the address.
    2^14 is the size of the host portion.
    2^14 = 2^8*2^6=256*64=(256*4)*(64/4)=1k*16=16K
    two of these addresses can not be used to address host.
    So, the number of usable host addresses is 16k-2

    number of subnets:
    2^18= 2^8*2^8*2^2=256*256*4=256*1k=256K

    which network does belong 

    looking at the /18, you can tell the first and second octets are part of the network and the fourth octet isn't.
    so, we have 10.20.xx.0
    The third octet needs to be calculated because a portion is the network address and a portion isn't.
    the /18 tells you the first 2 bits are part of the network address, which means the last 6 are used for hosts.
    that makes the last 6 bits of the network address of the third octet are zero.
    the first bit is 128 and the second is 64 and being the value is 10 which is less than 64 means neither bit is set.  
    and makes the first 2 bits zero.  so the third octet is 00 000000
    and the network is
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