# Can someone help with this classless subnetting of 10.20.10.18/18

Member Posts: 2 ■□□□□□□□□□
Can someone help with this classless subnetting of 10.20.10.18/18
Need how many usable hosts, howmany subnetwork comes and under which subnetwor this IP 10.20.10.18 comes..
I am specifically looking for the how many subnetwork comes under this prefix and how to calculate it.

• Member Posts: 1,772 ■■■■■■■■□□
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.
www.subnetting.net/

• Member Posts: 903 ■■■■□□□□□□
number of usable hosts:
/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
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 10.20.10.18/18 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 10.20.0.0