Subnetting Confusion

[tjmc]

Hello Everyone,

I have been studying subnetting and I can memorise all the powers of 2 (up to a certain amount before I have to start adding up) all the multiples of 16 etc...


I can also work out what the CIDR is from a subnet or subnet to CIDR etc...

I have just opened subnettingquestions.com and came accross the following question and realised that I have missed something really important in one of my steps of subnetting Class A and Class B networks.


Question: How many subnets and hosts per subnet can you get from the network 172.28.0.0/28?
I answered 14 hosts correctly, however initally looked at the /28 and thought it was a Class C network so immediately thought 16 subnets/networks.

However the correct answer is actually 4096 because it is a Class B address (i.e. 128-191).

So obviously the first thing I should be doing when subnetting is looking at the address and determine what Class it is in first.

This to me is a bit of a trick question as I would have thought the answer was 16 as the CIDR is /28?
Anyone have any pointers that I can keep an eye out for this sort of thing?

Thanks

[tjmc]

How come this is not coming up as a topic in the tcp/ip section?

[tjmc]

Hmmm.... never mind, it would seem that the forum is not sorted by default by date and time.

I still would like some help with regards to me question above though?

[Fuzz]

The class of network is important, not the CIDR notation. With that address, you know it is a class B address from the first octet, and therefore has /16 by default. Given the subnet /28, this means you have 28 - 16 = 12 subnet bits. 2^12 = 4096.

Consider this. If you subnet a class B address into equal sized /24 subnets, how many subnets do you have? The answer cannot be 1, as you have split the network down many times already. You have 256 /24 subnets in each /16 network address, and 16 /28 subnets in each /24. 16 * 256 = 4096 /28 subnets in each /16 network.

There's no trick to the question, you have to be clear what you are being asked, and not just assume. The best pointer I can give is read the question carefully.

[tjmc]

Thanks Fuzz, you cleared that up for me.

I kept looking back at the /28 and had the mindset that anything beyond /24 I treat like a C class, however I did not even think about the address ranges, even though I know what the address ranges for each class is.

So instead of thinking 12 host bits, I was thinking 4 instead of adding the other 8 bits from the B class.

Now that I look at my question, seems silly... hahaha.

Thanks

[anurag007]

CCNA Exploration v4 - Module 1 - IPv4 subnetting - Part 1
http://blip.tv/file/2416401/

CCNA Exploration v4 - Module 1 - IPv4 subnetting - Part 2
http://www.facebook.com/video/video....d=190010265716 (http://www.facebook.com/video/video.php?v=1220919762385&oid=190010265716)

u can also learn from movies

www.learntosubnet.com (http://www.learntosubnet.com/)

use these website that can help u in becoming pro in subnetting