Subnetting questions...need clarification

binaryhat

Hello,

I have a few Subnetting questions that I need help with:


Question: What is the first valid host on the subnetwork that the node 172.31.86.91 255.255.240.0 belongs to?


Answer: 172.31.80.1


256-240=16
3rd octect

16
32
48
64
80 -95
96


not 172.31.81.0...why? Is it because 172.31.81.0 is the first subnet?

Question: What is the first valid host on the subnetwork that the node 10.52.253.152/20 belongs to?
Answer: 10.52.240.1

/20
24-20=4
2^4=16
3rd octect


16
32
48
64
80
...
......
......
224
240 - 253
256


why 240 and not 241?

Also Is there a faster way to count by block size when dealing with a high number?


Question: What is the last valid host on the subnetwork 172.18.192.0 255.255.240.0?
Answer: 172.18.207.254

256-240=16
3rd octect


16
32
...
.....
.......
144
160
176
192 - 207
208


172.18.206.1


why 207? that's broadcast.

elderkai

1. Because it starts at 80, not 81. If it was subnetted into the last octet, yeah youd have to add one to the network number because you don't have room. However since it's in the third octet, even though 80 is the network number you still have the entire fourth octet for hosts. It would go from 172.31.80.1 all the way to 172.31.80.255 before switching to 81.

2. The response for the first answers this one.

3. Similar to the above. The reason it's not a broadcast is because you're working with a subnet that has more than one octet. The broadcast would be 172.18.207.255. The last usable would indeed be 172.18.207.254.

Hope that helped.

binaryhat

Thanks for the feedback, it helps. What about is there a faster way to count by block size when dealing with a high number?

elderkai

Not that I know of. You could always count by a larger number to get as close as you can. Say if the blocksize is 16, count by 32 to get as close to 240 as you can, then just subtract/add 16 to get there. That sounds like it'd work.