5th net of private class B when SM is 16?

Question

I need to calculate a network with VLSM. It says I should start with the 5th net of private class B when subnet mask is 16. What is this 5th net?

The network looks like this: https://dl.dropboxusercontent.com/u/34140308/uploads/IMG_20150108_074523.jpg

Well, do you know what is the first class B private network? — pajaja, Jan 22 '15 at 20:15
Yes, so the second is 172.17.0.0, 172.18.0.0 third etc... Also your question is offtopic: read this: http://stackoverflow.com/tour — pajaja, Jan 22 '15 at 22:01
Is this true even when SM is 16? Usually the SM of a class B network is 12, not 16. — Exceen, Jan 23 '15 at 06:46
@Exceen, Class B is /16. (Class A: /8, Class B: /16 and Class C: /24) — CustomX, Jan 23 '15 at 10:44
that's for public network classes, I'm talking about private classes — Exceen, Jan 23 '15 at 14:10
@Exceen, you are correct. I was mixing public/private. So for a private class B address using a /16 mask `172.20.0.0/16` is the fifth subnet (if I calculated correctly). — CustomX, Jan 23 '15 at 15:29
But why am I still allowed to modify those bits? Doesn't the subnet mask forbid this? I know that i's fine when SM is 12, I don't get the point why it would still be correct with SM 16 — Exceen, Jan 23 '15 at 16:07
@Exceen, because 172.16.0.0/12 is the entire range you can use. You are just subnetting in that range. — CustomX, Jan 27 '15 at 15:20
But I'm not talking about 172.160.0.0/12, I'm talking about 172.16.0.0/16 which has the range 172.16.0.0-172.16.255.255. — Exceen, Jan 27 '15 at 17:38

score 2 · Accepted Answer · answered Jan 30 '15 at 13:16

Exceen, it's easier if I write it as an answer.

The private class B network is 172.16.0.0/12.

You need to use subnet mask /16, so every private class B network is 0.0.255.255 large or a little more than 65k hosts per subnet.

1st subnet: 172.16.0.0/16
2nd subnet: 172.17.0.0/16
3rd subnet: 172.18.0.0/16
4th subnet: 172.19.0.0/16
5th subnet: 172.20.0.0/16.

You'll need to use 172.20.0.0/16 for your network diagram. From the diagram we can tell you need 8 subnets. So you can either split the 172.20.0.0/16 range into 172.20.0.0/24 - 172.20.8.0/24 or use VLSM to be more efficient.

Thanks a lot! I finally got it – Exceen Jan 30 '15 at 18:35 — Exceen, Jan 30 '15 at 18:35

5th net of private class B when SM is 16?

1 Answers1