Help with Subnetting Class B Addresses

[slaing]

Hi all,

I'm working thorugh chapter 3 and was good up until pg 130-131 where I got lost. For Practice Example 6B the subnet mask used was 255.255.255.128 & The valid subnets are 256 - 255 = 1. With Example 7B, the subnet mask used was 255.255.255.192 & valid subnets are 256 - 192 = 64.
I am confused as to why in example 7B, the 4th octect of the subnet mask was used to calculate the block size for valid hosts compared to example 6B where the 3rd octet of the subnet mask was used. Can someone please clarify why the 3rd octet of the subnet mask in 7B was NOT used to calculate block size for valid hosts?

:confused:

Thanks!

[tsunamoro]

Hi slaing,

I'll try to help you.

I hope that you are concerned that with "valid subnets" is indicated the incremental value used to write the subnet addresses.
Now (trying to be in Todd's mind, sorry Todd!), in example 6B, if you read carefully the sentences, it's said that for the third octect we have an incremental value of 256-255=1, which means that the numbers in the third octect increment by 1; so you have 172.16.0.XXX, 172.16.1.XXX, 172.16.2.XXX, ...., up to 172.16.255.XXX.
For the fourth octect the icremental value is 256-128=128. This means that the complete subnet addresses are 172.16.0.0, 172.16.0.128, 172.16.1.0, 172.16.1.128, 172.16.2.0, 172.16.2.128, up to 172.16.255.128.
OK? Both third and fourth subnet mask octects are used but just to go through the process, the implication of a 255 mask is analyzed first.
For example 7B, nothing changes for the third octec, but for the fourth now the icrement value (valid subnets) is 256-192=64.
This means that the subnet addresses are 172.16.0.0, 172.16.0.64, 172.16.0.128, 172.16.0.192, 172.16.1.0, 172.16.1.64, 172.16.1.128, 172.16.1.192, and so on up to 172.16.255.192.

Is it clear now?

In subnetting, you have to find the incremental value or, as it's said in the book, valid subnets. Read on page 119 - third bullet point.

The clue is, always look for the subnet octec mask that isn't 255; it's the value you have to use for determining the incremental value.

Hope this help.

[slaing]

Hi Tsunamoro,

Thanks for getting back to me.

Okay, so it appears that what you're saying is - you have to do the "256 - X" for BOTH the 3rd and 4th subnet mask octets when subnetting Class B networks, correct? I think part of my confusion stemmed from it not showing this being done for both octets in each example.

Thanks for your help!

[tsunamoro]

Hi slaing,

you are almost right (I'll tell you in a moment why only almost).

The address tell you the class of the network, then you know the default subnet mask. Knowing the default subnet mask you can know apply the subnetting procedure comparing your actual subnet mask and the default one.
For example, supposing you had a class B address; the default subnet mask will be 255.255.0.0; now let your actual subent mask be 255.255.255.192, comparing the two subnet masks you know that you have to apply the "256-X" rule to the third and fourth octects.

Now, why almost? Because you can get rid of default subnet masks and classes and enter to the world of classless inter-domain routing (CDIR) and you need to concentrate yourself only on the subnet mask and split the mask in the hosts part and subnet part and working with the "power of 2".
It's something you'll learn with VLSM.

Bye.

[dae26]

I still don't understand after reading these examples.

Under the Practice Example #7B Todd states that we handle this one the same way we would with a class C address because the third octect is a 255 mask and there is a subnet number in the fourth octet. Well, my big confusion is, how does example #7B differ from examples #5, and #6? They all have 255 in the third octect with a subnet number listed in the fourth octect.

Can anybody explain?

:confused:

[Sewer-rat]

Dae26,

When subnetting it is very crucial that you remember where your classful boundaries are at:

n= network bits h=host bits

CLASS A) n.h.h.h
CLASS B) n.n.h.h
CLASS C) n.n.n.h

The examples on those pages ARE class B subnets but with additional subnet bits turned on. Lets take example #7B 172.16.0.0/26.

Our DEFAULT mask for this since it is a class B would be 255.255.0.0 BUT here is where the subnetting part comes in!! The mask being used in the example is a /26 also known as 255.255.255.192. The subnet mask used in the example is still a class B subnet but we have taken 10 subnet bits out of the host portion of the address to create more subnets.

Defaul mask: 255.255.0.0 = 11111111.11111111.00000000.00000000
#7B mask: 255.255.255.192 = 11111111.11111111.11111111.11000000

If you apply the same logic we used above to the other examples you will see that we are doing the same thing for each one BUT we just borrow a different amount of host bits. Stick with it! I can remember when I was doing my net+ cert years ago and started to learn the basics of subnetting..drove me nuts! LOL In the end once you understand it and can do it off the top of your head it becomes extremely critical in mastering the rest of the topics in the CCNA course. IF you want extra practice try this site:

http://faculty.valleycollege.net/rpowell/jscript/subnet2.htm

It generates random subnets and makes you identify the network address, host ranges and broadcast. Hope we cleared a few things up for ya. Best of luck! :)

[dae26]

Thanks! That information was helpful as was the link to the subnetting practice site. I think I am finally getting it (and I graduated with a degree in IT over a year ago. I know, it's embarrasing!)

Thanks again!

:)

[Sewer-rat]

Np! Glad it cleared things up & enjoy the link :)

Don't be embarrassed ;) I imagine they wanted you to understand subnetting during your schooling but probably not at the extent that the CCNA requires.