Subnetting class B

agtjamesb007

There is plenty of information out there on sub-netting a class C address, but what about a class B?

I have a particular question in my study book that I just cannot seem to understand how they came out the answer.

The question reads: Is 172.16.255.0/18 a valid host?
The correct answer is yes; the subnets valid host IP's range from 172.16.192.1 to 175.16.255.254

However when I look at the question, I see that we borrowed 2 bits. 2^2 = four sub networks with an increment of 64.

IP 172.16.11111111.00000000
Default Mask 255.255.00000000.00000000
Subnet Mask 255.255.11000000.00000000

Ranges 172.16.192.0 - .63
.64 - .127
.128 - .191
.192 - .254

Now since its a class B I dont know how the third octet becomes 255, but I have four subnets, even though the answer only lists one.

I think our particular IP lands in a range where 172.16.255.0 appears to be a network address.

Can anybody help? I hope i provided enough information to my troubles.
Thanks in advance!

EdTheLad

Subnets

172.16.0.0/18 host range 172.16.0.1 -> 172.16.63.254
172.16.64.0/18 host range 172.16.64.1 -> 172.16.127.254
172.16.128.0/18 host range 172.16.128.1 -> 172.16.191.254
172.16.192.0/18 host range 172.16.192.1 -> 172.16.255.254

mikeybinec

The guru above laid out your address ranges. One thing folks get confused when subnetting is they think a 0 can only be a network address and a 255 can only be a broadcast address. The Class B range has hundreds or more hosts in it's own subnet or club

mella060

You have to remember that the increment/block size is in the 3rd octet, not the last one.

So the networks would go up by 64 in the 3rd octet...

172.16.0.0/18
172.16.64.0/18
172.16.128.0/18
172.16.192.0/18 host range 172.16.192.1 - 172.16.255.254 broadcast address 172.16.255.255

So 172.16.255.0 belongs in the last network

Jon_Cisco

I think this question is designed to make you think about how the address blocks work. You learn about Class C addresses first and most people tend to think that 0 is the network and 255 is the broadcast. This is only true at the start of your study before you get into subnetting.

Once you start subnetting you will learn your address blocks can contain these numbers and this question emphasizes that fact.

tecnodog7

So as the masters have spoken and explained beautifully I am still going through with sub netting the old fashion. With a paper and pen. So i actually worked out the example. Thank you giving me an opportunity to practice my sub netting. Hope this helps.

agtjamesb007

tecnodog7 wrote: »

So as the masters have spoken and explained beautifully I am still going through with sub netting the old fashion. With a paper and pen. So i actually worked out the example. Thank you giving me an opportunity to practice my sub netting. Hope this helps.

Thank you everybody for helping me out. And thank you Technodog for working it out!
Makes more sense now. I suppose it threw me off that the answer said "The valid host IPs range from 172.16.192.1 to 172.16.255.254" without listing the individual ranges. I was misinterpreting the answer.

tecnodog7

Dude all I can say is you just need to practice. I have done about 100 different sub netting problems with a mixture of class A B C, including VLSM (Same concept more work).
But yea for us noobies we need to do things with a pen and paper until the time we become like EdTheLad who can just look at it and give you the answer.

Jon_Cisco

subnetting.net - Subnet Questions and Answers

I can't recommend this site enough. Spend 15 minutes a day on it and you will be doing this stuff quickly in your head in a couple weeks. For the test subnetting will become easy points.

tecnodog7

Jon_cisco, thanks for the website. Even though i feel comfortable with subnetting, there is no such thing as knowing it too good. I need to make sure i score 110% out of 100 for subnetting on my ICND 1

RouteMyPacket

One thing you need to forget about is IP classes, they don't really exist anymore. It's absolutely irrelevant and has been for a long time.

Subnetting is subnetting regardless of "class". CIDR is what we care about and use. Play with it if it's needed to pass an exam but other than that, meh forget it.

So what classes would these be?

172.31.10.0/24
10.10.15.0/26
192.168.70.0/16

agtjamesb007

I know classes are unused, in fact they teach so much stuff that isn't used anymore. They still talk about hubs. But arent classes important when subnetting so you know how many bits were "borrowed"?

EdTheLad

You don't need to know how many bits are borrowed, that concept always sounded so silly to me. You have a mask and that tells you how many bits are used for the host and how many are used for the network. Now that everything is classless, i.e. "ip classless" is enabled by default on all ios 12.2 and above you don't need to really worry about classes. The only place i can think of where classes come in to play is when using ripv2 summary, ripv2 doesn't support a mask less than the major network. Are there rip networks still out there? possibly