How Many Subnets/Host Speed Practice 1

Friday (ok, well, late Thursday) this week for the subnet speed practice. Today’s variation: find the number of subnets possible, and number of hosts per subnet.

Suggestions BEFORE CLICKING to see the rest: be ready to start your timer. Be ready to finish all 5 problems.

Your job:  Based on a class A, B, or C network number, and a mask, with the assumption that the mask is used for all subnets in the network

1. List the number of subnets that exist
2. List the number of usable host IP addresses per subnet

You may assume that the zero and broadcast subnets are usable.

1) 9.0.0.0/17

2) 100.0.0.0/21

3) 151.151.0.0/22

4) 201.201.201.0/27

5) 223.223.221.0/30

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[…] last Friday’s speed practice? Sorry for the delay in posting the answers. They are below the fold. As usual, I’ve listed […]

Hi Wendell,
1) 9.0.0.0/17 – for this address, the mask will be 255.255.128.0, the no. of subnets i got is 2. But your answer says, 512 subnets. How is it possible. Can u explain in detail. Thanks in advance.

Regards

Vishnu

Hi Vishnu,
I think you’re forgetting that the IP network is a class A network, which means it has 8 network bits. With a prefix of /17, there are 9 (17 – 8) subnet bits, and 2^9 =512. Maybe you were thinking of class B, with 16 network bits, leaving 1 subnet bit?
Wendell

Hi Wendell,
I’m currently working through Chapter 15 in ICND1 100-105.
I’m currently reading about dividing a Classful into 3 parts and I’ve always been under the impression a Classful network is divided into 2 parts, Network and Host.
I think I may have finally understood this mistaking I’m making. Can you please confirm whether my assumption is correct..

A classful IP address e.g. Class A has a default subnet mask of 255.0.0.0 but may always borrow host bits to create subnet bits and still remain classful.

It’s only when the Class uses less than it’s default subnet mask. e.g. a 192.168.0.0/16 network would be classless but a 10.0.0.0/16 would be classful.

Is this correct?

Hi Sam,
Actually, I think the struggle is in your use of the words “Classless” and “Classful”. Couple of points:

Many people in the industry use these terms to mean different things. So you may have learned something different from what I state in the book.

There’s no standards doc that I know of that defines the use of the terms (and I’ve looked).

I think to give you a full answer I’d be repeating what I wrote in the book. I don’t say that to just defer answering, but if you’re searching for a better understanding, I’d say to ignore all you have learned about the terms, and re-read that chapter without using any concepts you learned before about those terms. That’s probably the biggest hurdle.

The book of course gets into more detail.
Key here is that classless/full addressing is a way to think; it’s not an attribute.

Then, classless/full routing is a Cisco router “feature” having to do with the use of default routes

…and
classless/full routing protocols is an attribute of routing protocols, whether they do (classless) or do not (classful) advertise subnet masks with their routing updates, therefore supporting VLSM.

Hope this helps!
Wendell

Hi Wendell,

Does adequate speed performance on this drill require memorization all powers of two up to 2^24? I know the first twelve or two, but not the next 12, so if they ask how many hosts in a /10 block or subnets in 10.0.0.0/27 I’d have to do time-consuming arithmetic.

Hi Peter,
Couldn’t really speculate on the specifics of what is or isn’t on the exam. I could say though that I’ve been told by Cisco Cert people over and over that the goal isn’t to trick you on trivia. EG, I wouldn’t expect you to see 4 answers for a value that is 2^22, with the 4 answers being 2 more or less than the previous answer, to test whether you had memorized the powers of two. Much more likely is whether you can do the math to find subnets, determine what addresses are in a subnet, that kind of thing.
If it were me, I’d memorize at least through 2^16. But that’s not some secret special insight – just an opinion.
Wendell

4:17 to complete the lab

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