VLSM Clarification

Hi Guys,

Can you just confirm if I have this question (that I set myself) correct?

Q. Subnet 172.16.0.0 to address the below network using the most efficient addressing possible

I have come up with the following VLSM's:

172.16.1.0 - 172.16.1.127 /17 Network for 113 users
172.16.1.128 - 172.16.254 /17 Network for 93 users
172.16.2.0 - 172.16.2.31 /19 Network for 16 users
172.16.2.32 - 172.16.2.35 /30 Network for Wan Link
172.16.2.36 - 172.16.2.39 /30 Network for Wan Link
172.16.2.40 - 172.16.2.43 /30 Network for Wan Link

I think this is right, but just wanted to make sure incase I have done anything daft.

Many Thanks
Nick

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billscott92787

surfthegecko wrote: »

Hi Guys,

Can you just confirm if I have this question (that I set myself) correct?

Q. Subnet 172.16.0.0 to address the below network using the most efficient addressing possible

I have come up with the following VLSM's:

172.16.1.0 - 172.16.1.127 /17 Network for 113 users
172.16.1.128 - 172.16.254 /17 Network for 93 users
172.16.2.0 - 172.16.2.31 /19 Network for 16 users
172.16.2.32 - 172.16.2.35 /30 Network for Wan Link
172.16.2.36 - 172.16.2.39 /30 Network for Wan Link
172.16.2.40 - 172.16.2.43 /30 Network for Wan Link

I think this is right, but just wanted to make sure incase I have done anything daft.

Many Thanks
Nick

First let me start by saying, that I don't mean this offensively but, you should definitely review your VLSM subnetting and subnetting itself. Your masks for your first three examples are way off. I provided an step by step break down for you. Please ask me if you have any questions, I'll be glad to answer them.

If you require 113 users, lets start with 172.16.1.0. 113 users requires that you have at least 7 bits for the host portion 2^7 = 128 - 2 = 126.

So, technically, (I'm confused on your addressing scheme). If you network that you are given to start is lets say 172.16.1.0 /24. You could do the following:

113 users -> 172.16.1.0 /25 this would leave 7 bits for the host portion and the range would be:

172.16.1.1 - 172.16.1.126 (.127 would be the broadcast)

Next, you would move on to Network for 93 users: You have to determine how many bits. Well the same thing that we just used. 2^7 = 128-2 = 126 hosts

93 users -> 172.16.1.128 /25 this would leave 7 bits for the host portion and the range would be:

172.16.1.129 - 172.16.1.254 (.255 would be the broadcast address)

Network for 16 users. You would use 2^5 = 32 - 2 = 30 hosts. Since (2^3=16 but when you subtract 2 addresses that are reserved it only leaves you with 14. 2^3 = 16-2 = 14)

172.16.2.0 /27 which would give you a subnet block of 256-224 = 32

IP Range:

172.16.2.1 - 172.16.2.30 (.31 would be the broadcast address)

WAN Links:

172.16.2.32 /30 (this is derived from above since the next subnet available would be 172.16.2.32/27) Subnet Block = 256-252 = 4 (252 comes from /30)

IP Range:

172.16.2.33 - 172.16.2.34 (.35 broadcast)

2nd WAN Link:

172.16.2.36/30

IP Range:

172.16.2.37 - 172.16.2.38 (.39 broadcast)

3rd WAN Link:

172.16.2.40/30

IP Range:

172.16.2.41 - 172.16.2.42 (.43 would be the broadcast)

I highly recommend you go back and review VLSM. You are trying to subnet in the 3rd octect, when you don't have to. You would concentrate really on the 4th octect in this one. VLSM's purpose is to efficiently use the available address space that you are given. You had the right idea as far as addressing goes, but your subnet masks for the subnets themselves are way off. Now your WAN links are fine as far as the broadcast. But the way you posted you make it look like you can use .43 in the last WAN link and you can't. Practice, Practice, Practice, is the only thing that I can recommend

Here is the reason that I say these masks are off:

172.16.1.0 - 172.16.1.127 /17 Network for 113 users

/17 mask is 11111111.11111111.1000000.00000000 or 255.255.128.0 ( which would give you a block size of 256-128 = 128 ) In this example you are concentrating on the 3rd octect, which technically would give you this addressing:

172.16.0.0 (say this is the starting address /16)

172.16.0.1 - 172.16.127.254 (.255 would be the broadcast) See how the it is concentrating in the 3rd octect. With this example you are using 15 host bits (0's) which means 2^15 = 32,768 - 2 = 32,766 hosts. So, you are wasting 32,640 addresses including the reserved ones.

172.16.128 - 172.16.254 /17 Network for 93 users

You used the same mask again /17 mask is 11111111.11111111.1000000.00000000 or 255.255.128.0 ( which would give you a block size of 256-128 = 128 )

What this means based on your subnet mask is you would use the following addressing:

172.16.128.1 - 172.16.255.254 (.255 would be the broadcast for this subnet). You are using the same amount of space as the example above.

172.16.2.0 - 172.16.2.31 /19 Network for 16 users

As you can see in the above example you have used the entire space of 172.16.x.x This would then round up to

172.17.0.0 /19 (in your example) /19 is 11111111.11111111.11100000.00000000 or 255.255.224.0 (This would give us a subnet block of 256-224 = 32) So what this means is you would use the following addressing:

172.17.0.1 - 172.31.255.254 (.255 would be the broadcast address) In this example you have a total of 13 host bits (0's) 2^13 = 8192 - 2 = 8190. This means that you are using 8,190 host addresses and 2 reserved for a total of 8,192 - 16 = 8,176 addresses wasted if you only need 16 hosts.

I highly recommend you go back and review subnetting and VLSM as well. Since you were concentrating on the 3rd octect, this is a skill set learned with just subnetting itself really. Go back hit them both again, and practice, practice, practice. Also, don't sweat the fact that you need to review it. Many people struggle with getting subnetting down. I've talked to people that work in my networking department that have issues with subnetting and VLSM. Some of them I have asked questions about VLSM don't even know how to do it. Scary huh :0 I know. Some of them aren't even certified. They have been doing it for a long time and know how to troubleshoot, and know what to do for their job though.

Xenz

Way to make it look pretty, who the hell gave you a CCNA anyways? Why did this happen or when

I'm gone for one week and this is what you've done.

billscott92787

Hey man

lol. See what happens when you up and leave and don't post for a week or answer my PM's? LOL just kidding. Some guy just walked up to me on the street and said, "Hey do you want your CCNA." I told him sure and that was it. LOL Just kidding, I took the test this past Thursday and passed, I was so relieved. I have never been so nervous in my life when taking the exam.

surfthegecko

Dont worry, I obviously wasnt concentrating whilst doing this (was watching a video on VPN's.

Ended up playing with the wrong octect and then typed it up as I had written it down.

Got the wan links right (which was when the video ended).

Just goes to proove that men cant do more than one thing at a time.

With regards to broadcast, I know the first and last one in each subnet is the network/broadcast address, and should have clarified that I was in fact too lazy to included the extra line (that I would normally write down), so this is my fault.

It is a good lesson in how to embarase yourself and remember to pay attention when in an exam.

Thanks for the response though it was good to just read through it for clarification sakes anyway.

I will try a couple more now just to be on the safe side.

billscott92787

No problem man. I wasn't trying to embarrass you. Subnetting, definitely is something that can't be multi-tasked though. LOL. Men can do more than one thing at a time, I do it at work all the time. But, subnetting requires full concentration. I think you have an understanding because you got the WAN links right. As you said you concentrated on the wrong octect on the other examples, just review it, do a few more, and then make sure you understand it. You passed the ICND1 so you have to have an understanding of it. Everyone makes mistakes and what does it do? It makes you go back and see where you messed up, correct it in your brain, and then move on. No one is perfect, I definitely am not, you aren't no one is.

surfthegecko

lol... on a less serious note I did like the fact that I mentioned "using the most effective addressing possible", and then went and wasted 32,640 hosts on the first couple of subnets.

surfthegecko

billscott, I use this as an excuse to get out of doing stuff all the time.

Women always say that men cant multi task, so I then use this to my advantage when Im being asked to do multiple things around the house, they cant argue it either because women are normally the ones that pass the comment in the first place....

billscott92787

I totally agree.

LOL I won't tell if you don't tell. LOL