Subnet Question - Finding Valid Host Ranges Fast

shortcut144

So I am working on subnetquestions.com and trying to speed test myself through the questions. I've got one style of question that takes me a long time to answer. Wondering if you guys know a shortcut.

What valid host range is the IP address 172.20.156.105 255.255.240.0 a part of?

Now based on the subnet mask I know that I will need to increment the 3rd octet by 16 (Last network bit of the subnet mask). So now I proceed to cound up from 0 to go up to 156 and find the range. This takes a long time for me as I am preparing without a calculator.

Any suggestions to speed up finding the ranges?

jamthat

The way I do it in my head is:

- increment of 16, need to find where '156' might fall..I'll try to find a multiple of 16 that is close to 156
- start with the easiest: 16x10 = 160...there you go! 160-16 = 144
- 172.20.144.0 is your network address, leaving the host range to be 172.20.144.1 - 172.20.159.254. broadcast will be 172.20.159.255

Hopefully this helps..I can do it extremely fast in my head but have a tough time explaining how. Feel free to throw more questions at me and I'll do my best

iamme4eva

I usually start counting up from a common number that I know - if I was doing 156 in that one, I'd go for 128 and count up.

However suits you best, some numbers just stick in your head.

Carnby

iamme4eva wrote: »

I usually start counting up from a common number that I know - if I was doing 156 in that one, I'd go for 128 and count up.

However suits you best, some numbers just stick in your head.

Yes, 128 and 192 are safe places to start counting up from. (Obviously only if the address is higher than 192)

Carpe Porcus

Convert both values to binary and "and" the two values (1+1 =1 with 0+1 or 1+0 and 0+0 all = 0)

IP Address: 172.20.156.105 = 10101100.00010100.10011100.01101001

SNM: 255.255.240.0 = 11111111.11111111.11110000.00000000

IP Address: 10101100.00010100.10011100.01101001
SNM: 11111111.11111111.11110000.00000000

And value: 10101100.00010100.10010000.00000000

10101100.00010100.10010000.00000000 = 172.20.144.0

This is an alternative for not using a calculator, takes time but with practice you can get the answer quickly.

iamme4eva

That's the thorough way of doing it Carpe, but not really one you can do quickly in your head! Think the OP was looking to speed his subnetting up - finding the increment of 16 and doing 128+16 in your head should be a lot quicker than all that binary conversion.

Unless you are a binary machine and can just smash that stuff out - it would take me ages to work out the binary for all of that.

Carpe Porcus

I agree, it isn't something you can do stright away it does take practice but once you get used to it you get the required subnet from this method. Like everything else, once you flex the brain you can do it in your head and without a calculator (as OP stated) it is a great way with pen and paper.