1. ## simple subnetting

I have a networking exam coming up and i was going over some past papers. came across this question

1. Given the following host IP addresses 172.16.20.36/28:

(i) What is the subnet mask (in decimal) for this network?
(ii) How many bits have been borrowed to create this subnet?
(iii) What is its network address of this subnet?
(v) What is the usable host address range for this network?
(vi) How many usable host addresses are on this networks?

(i) 255.255.255.240
(ii) 4
(iii) 172.16.20.32 \28
(iv) 172.168.20.47 \28
(v) 16
(vi) 128

thanks

2. Note, I'm a bit rusty and not a networking expert. I'm like 95% sure of the following:
(i) Correct
(ii) Correct
(iii) Correct
(iv) Correct
(v) Wrong. What is the usable host address range for this network. You are wrong in two ways here, hence the color coding. See (iii) and (iv). Not all addresses in the range are usable for hosts. Also, you did not provide an actual address range, only (I presume) the number of addresses in that subnet.
(vi) Wrong. I'm not sure where you got that number (please explain). However, if you consider (i) through (v) carefully, you'll notice that there are not 128 addresses in that subnet. If you get (v) correct, you can literally count the numbers in the range to get the answer here.

3. thanks for (v) i have misread the question it appears . i did give the amount of addresses in that subnet. the range in which the given ip address is 172.16.20.33--->172.16.20.46 but how to get the entire range from that.

for part (vi) i got 128 from doing this 8 subnets with 16 addresses and 14 useable address in each gives 128 useable addresses?(*edit this is also wrong 8 * 14 != 128(sigh))

would it be 256 - 16 = 240 usable addresses?

i kinda knew i had the last 2 wrong. but in class and labs we we give an ip address and told that it needed X amount of subnets- which i can do but in the exams the subnetting question seems to be in this format which i find quite confusing..

4. Forgot this in my first post...at least, I think this is true:
You typically don't provide the number of netmask bits when giving the network or broadcast. And if you did, traditionally you use a / forward slash, not a \ backslash. Minor issue when answering on paper, critical when using network tools, writing network config files, etc.

Originally Posted by alien1
thanks for (v) i have misread the question it appears . i did give the amount of addresses in that subnet. the range in which the given ip address is 172.16.20.33--->172.16.20.46 but how to get the entire range from that.
That looks right.
Originally Posted by alien1
for part (vi) i got 128 from doing this 8 subnets with 16 addresses and 14 useable address in each gives 128 useable addresses?(*edit this is also wrong 8 * 14 != 128(sigh))

would it be 256 - 16 = 240 usable addresses?

i kinda knew i had the last 2 wrong. but in class and labs we we give an ip address and told that it needed X amount of subnets- which i can do but in the exams the subnetting question seems to be in this format which i find quite confusing..
Perhaps there was a translation/copy issue. Question (vi) is not grammatically correct, this implies singular while networks implies plural. I assumed you meant a single network, that the question should have read this network. You apparently think it means these networks, as in all the networks under the 172.16.0.0/12 private network. Which is it?

5. Originally Posted by anduril462
Forgot this in my first post...at least, I think this is true:
You typically don't provide the number of netmask bits when giving the network or broadcast. And if you did, traditionally you use a / forward slash, not a \ backslash. Minor issue when answering on paper, critical when using network tools, writing network config files, etc.

That looks right.

Perhaps there was a translation/copy issue. Question (vi) is not grammatically correct, this implies singular while networks implies plural. I assumed you meant a single network, that the question should have read this network. You apparently think it means these networks, as in all the networks under the 172.16.0.0/12 private network. Which is it?
So for the last question I think it means the total amount of useable address on each subnet.my reason for this is that it would have been easier to ask how many useable addresses on this subnet?answer 14.
Thanks again

6. Oops, I missed this in my first post. (iv) is wrong, but I'm sure it's just a typo. You have 172.168, it should just be 172.16.

As for (vi)

Yes, if it were just "how many useable host addresses are on this network", it would be trivial.

However, if it's "how many useable host addresses are on these networks", it gets a bit more complicated.

It sounds like you are referring to private networking, and looking at all the subnets that fill 172.16.0.0 - 172.31.255.255. That would require you to make an assumption not stated anywhere in the problem: the size of all the other subnets. There's nothing that prevents you from having several /4 subnets, as well as some /6 or /8 subnets. The answer to (vi) would then depend on the actual breakdown of subnets since the size of the subnets affects how many will fit in the overall /12 space, and the number of subnets affects how many of those addresses are reserved for network and broadcast, and thus unusable as host addresses.

If you really are meant to assume that it is entirely full of /4 subnets, then your problem should be fairly simple. How many total addresses in the /12 space? How many usable host addresses in one /4 subnet? How many /4 subnets are in a /12 space?