VLSM and summarization

[mrworldwarrior]

Hi Todd,

I prepare to take CCNA exam by next week... Here i need some tips. how to do subneting, vlsm, or summarization with save time, without make covert the decimal to binary...

Example

Let say summarized block like below how to get the subnets or the valid ip addresses from it ?

1-192.22.144.0/20?


2-192.25.33.0/25?

need some save time in exam......

I have read Todd book CCNA Study Guide, in VLSM and Summarization in Chapter3. Pages 137 to 149. But i still not really nail in mind.....

[Fuzz]

Have a read of these posts I made on my blog. This topic has also been covered in great detail on this forum, there are countless posts about it. Just use the search!

http://altarespot.lessergods.eu/?p=7
http://altarespot.lessergods.eu/?p=13
It's difficult to 'reverse engineer' a summarised block, because there is no way to tell where the denominations are. You can only really summarise known subnets, or subnet a larger address into smaller networks.

[mrworldwarrior]

VLSM, How to do summarization for many time of network or subent?

Example

192.168.10.5/25
192.168.10.0/26
192.168.1.26/28
192.168.1.27/28

[MiniMatt]

It's like riding a bike, after a while it becomes second nature; it also becomes quite hard to remember how you do it as you just do. I think one method of speeding things up is to just drill them into your memory in the same way as you may (or may not, dependent upon prevailing education attitudes at the time) have learnt times X tables as a kid.

Try remembering just one halfway block and then work from there.

You know that 172.16.x.y /16 is a regular unsubnetted class B
You know that 192.168.10.x /24 is a regular unsubnetted class C.

Remember 4 bits of subnetting for each of these. So:
172.16.x.y /20 = 255.255.240.0
192.168.10.x /28 = 255.255.255.240

Then for any questions with different subnetting you can just walk forwards or backwards from your memorised positions. A subnet will always be:
128, 192, 224, 240, 248, 252, 254 (and you won't see the last one on a class C address)
So if there's one subnet bit more than your memorised position go up one, one less, go back one.

Once you've got the subnet mask figured it's easy. I still use the method Todd's been teaching in his book since at least the 3rd edition. Take the interesting subnet octet and subtract it from 256 to get the block size. So with a subnet mask of 240 you know the block size is 256-240 = 16. You can then start walking through the available network addresses, remembering that the first address (eg. 192.168.10.16 /28) will be the network address and the last address before the next block will be the broadcast address (eg. 192.168.10.31 /28); anything in between is available for hosts.

And failing all that, the time available on the CCNA exam is quite generous. It's vital that you completely understand subnetting (and the closely associated wildcard masks) - a thorough understanding of it is really non-negotiable for the exam, however it doesn't matter too much if you can't do it quickly just yet. You're given a dry wipe board and marker when you go in (not allowed to start scribbling on it till you start the exam though) - so you can always put a memorised list at the top of that to help you through it. If you can ascertain the number of bits used for subnetting, then quickly count off that number of bits along a line:

128, 192, 224, 240, 248, 252, 254

... you'll be fine.

[MiniMatt]

VLSM, How to do summarization for many time of network or subent?

Example

192.168.10.5/25
192.168.10.0/26
192.168.1.26/28
192.168.1.27/28

Ok, so let's work out the available host ranges for each one:

192.168.10.5 /25 = 255.255.255.128 so block size of 128, net address 192.168.10.0, broadcast 192.168.10.127
192.168.10.0 /26 = 255.255.255.192 so block size of 64, net address 192.168.10.0, broadcast 192.168.10.63
192.168.1.26 /28 = 255.255.255.240 so block size of 16, net address 192.168.1.16, broadcast address 192.168.1.31
192.168.1.27 /28 = 255.255.255.240 so block size of 16, net address 192.168.1.16, broadcast address 192.168.1.31

So, you've got an odd (and decidedly huge) range to summarise there, and in production you'd have to make sure that you're not including any other networks that shouldn't be summarised (like 192.168.5.x) but from the above you see that the lowest possible network address is 192.168.1.16 and the highest possible address (a broadcast address) is 192.168.10.127.

Now it should be apparent that your subnetting is going to have to be in the 3rd octet, because that's the bit that encompasses the span from .1 to .10. Now you know that from subnetting, there are no subnets that provide a block size of 10. The only block size that would cover all that would be 16. 256-16 =240 so your summary subnet could be 192.168.0.0 /255.255.240.0 (or 192.168.0.0 /20). That would cover a huge range of other addresses in the summary as well (ie. it would also include other networks like 192.168.0.0 /24, 192.168.5.0 /24 and 192.168.14.0 /24).

[Fuzz]

VLSM, How to do summarization for many time of network or subent?

Example

192.168.10.5/25
192.168.10.0/26
192.168.1.26/28
192.168.1.27/28

You don't summarise just any old addresses, you need to make sure you summarise only the networks that require summarising without including those you do not want summarised. The addresses you have given here are a a bit random. You have one subnet address and three host addresses, you can only summarise subnet addresses.

If I subnet a class C address, 192.168.10.0 into 4, I could have 192.168.10.0 /25, 192.168.10.128 /26, 192.168.10.192 /27 & 192.168.10.224 /27. If I wanted to summarise all of these addresses I would use the original network address of 192.168.10.0 /24. If I only wanted the last 2 addresses summarised I would use 192.168.10.192 /26. See how the summaries are connected to the original addresses?

[mrworldwarrior]

Thanks you for support....

What the realy benefit of VLSM and summarization in our network....
My network exprience, subneting is one of most challenging to face... I am extualy new in networking. Sorry if i keep asking the same topic..

[Big Evil]

http://www.youtube.com/watch?v=YO6UUpzJ5-M

Should help, from the man himself.

[MiniMatt]

Thanks you for support....

What the realy benefit of VLSM and summarization in our network....
My network exprience, subneting is one of most challenging to face... I am extualy new in networking. Sorry if i keep asking the same topic..

The benefit of VLSM is more efficient utilisation of available addresses. The benefit of summarisation is reduced processing for routers, reduced size of route advertisments and the like.

Say we have only a single class C address available to us, 192.168.10.0 /24. Suppose we wanted to split that into three subnetworks but they were all different sizes - one has 120 hosts, the other two have 50 hosts each. Without VLSM (Variable is the key word) we'd have to build each subnet to accomodate the largest one - so each subnet would have to be a 128 block, and we can't fit three of those into a single Class C address range. But with Variable subnets we could use 192.168.10.0 /25 to accomodate the 120 users and 192.168.10.128 /26 & 192.168.10.192 /26 to accomodate the two blocks of 50 users.

Now upstream routers could be told "hey, I've got routes for networks 192.168.10.0 /25, 192.168.10.128 /26 and 192.168.10.192 /26 - send all your packets for these networks to me!". Or, much more efficiently, it could just say, "hey, anything for 192.168.10.0 /24 send to me". This way, upstream routers don't have to know anything about the three subnets, they just know that a router over there handles anything that falls into the range 192.168.10.0 /24, they don't have to look through three lines in their routing table before they forward a packet, they only have to look at one.