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Need Help with VLSM Subnetting Class A and B

HI cisco community

Ive just started doing my first CCNA course

I can subnet a Class C with variable length with no problems, but when I get to class B and A I really get lost

I can also subnet a Class ABC classfully as well

But Im having trouble with subnetting classless A and B

My instructor has taught us a pretty easy method of doing it via binary table method

So Id like to stick to that method if possible, Does anyone know any Binary table methods to subnet a class A or B with VLSM ?

3 REPLIES

Need Help with VLSM Subnetting Class A and B

Do you have any questions specifically related to what problem you're having? It's done the same way that you do for a class C except the amount of bits used obviously change.

If I have 172.16.10.0 255.255.0.0, that's a class B address and I have networks 172.16.0.0 - 172.16.255.255 to use.

If I want to break that down further, I have to borrow bits:

We'll work with the 3rd octet in the mask. Currently, I have 2^16 (65536) and that gives me 65536 hosts and 65536 subnets to work with. Let's say that I want to add more networks and less hosts. I'd go to the right and borrow bits, so now I have 2^18 (borrowed 2 bits from the third octet) and now I have 2^18 (262144) subnets and 2^14 (32 - 18 = 14) 16384 hosts.

That changes my subnets available and my mask now. At 2^18, my mask will be 255.255.192.0 which now means that my networks available are:

Mask originally:

11111111.11111111.00000000.0000000

New mask:

11111111.11111111.11000000.0000000

The 2 bits that I borrowed stop on 64, so my networks increment by that:

172.16.0.0 255.255.192.0

172.16.64.0 255.255.192.0

172.16.128.0 255.255.192.0

172.16.192.0 255.255.192.0

But, you can break these down even further. If I wanted to take the network 172.16.128.0/18, and break it down to 4 more networks, I'd borrow 2 bits (2^2 = 4) and make my mask 255.255.240.0. The boundary is 16 because of the 2 further bits that I borrowed from the original mask:

Original mask:

11111111.11111111.11000000.0000000

    255         255          192            0

New mask:

11111111.11111111.11110000.0000000

    255          255          240          0

1st network: 172.16.128.0 255.255.240.0

2nd : 172.16.144.0 255.255.240.0

3rd : 172.16.160.0 255.255.240.0

4th : 172.16.176.0 255.255.240.0

HTH, John *** Please rate all useful posts ***
New Member

Need Help with VLSM Subnetting Class A and B

Thanks alot for your help

I was able to do a Class B VLSM just this morning, I had to write out the whole table though with both octets

Just 1 more question I have, there is only about 3 weeks to go before the internal exams for my CCNA Module 1 Network Fundamentals course

I have been reading alot but there is so much to cover in such short time, Im currently up to reading chaper 5, theres 11 chapters in total on the online curriclum. Im not sure if Im going to have time to read all of it before the exam. Does you recommaned any specific chapter I should definently cover?

Thanks

Need Help with VLSM Subnetting Class A and B

I would reschedule the exam if you can. It's been a long time since I've done CCNA, so I don't remember how in-depth they get on the exam or what they cover. The exam has also changed since I took it. The best advice that I can give you is that you can't go back to any questions. If you click next after answering a question, you're not allowed to hit the back button; the question is permanently answered. You're going to need to know all of the material and not just hyperfocus on one or two chapters unless you are already very comfortable with the other material. If you haven't went through the other remaining 6 chapters, you're going to be missing a lot of material. My recommendation is to postpone your test until you're comfortable. With that being said, you'll know when you're ready.

HTH, John *** Please rate all useful posts ***
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