VLSM Question

kweevuss

Im going to be taking my CCNA exam Thursday, and the only thing that is honestly holding me up is VLSMs. I understand the blocksizes for class C, (well I think ) but after that I'm not sure.


What summary address would be sent from router A?

RouterA:

3 networks attached:
172.16.128.0/18
172.16.32.0/24
172.16.64.0/18

And its going to send this to router B as a summary address.

The correct answer is 172.16.0.0 /16 from what I am reading, but I don't understand how that is coming about, any help? Thanks alot!

PS. Is this something really important on the CCNA?

sratakhin

I do it this way:

Imagine that you have hosts 172.16.32.0, 172.16.64.0 and 172.16.128.0. What subnet mask would you have to use if you wanted to put them on the same subnet? Most certainly, it can't be /18 because you could only use one of the given addresses. If you tried /17, you could fit either 32.0 and 64.0 or 128.0 in the subnet. If you used /16, then all three would be on the same subnet.

YFZblu

Yep - Using the /17 subnet would fail to encompass everything:

172.16.0.0 - 172.16.127.255

I think this question is confusing because it undermines the spirit of subnetting, which is to conserve IP addresses. At least that's my understanding of it. By using /16 there is lots of wasted address space. Most likely in the exam you'd have a much tighter fit in terms of choosing subnets that don't waste tons of address space.

kweevuss

Ok so I think it might have just clicked, I basically need a subnet that would have room from the address 172.16.32.0-172.16.128.0? And thats the whole point and to obviously find one that doesn't waste a ton of IPs

sratakhin

That's right.

kweevuss

Ok thanks alot! I am going to have to pound this away for a while longer to get this good lol

synseq

VLSM is a lot more efficient than classful subnetting. For instance say you borrow 4 bits on a class C network that gives you 14 valid subnets with 14 valid hosts. If you have a serial link to another router(which only require 2 hosts) you end up wasting the other 12 hosts. With classful subnetting you use the formula 2N(where N is the number of network bits) - 2 to get the number of valid subnets. With VLSM you can use the ip subnet-zero command then you can just use 2N and utilize those other 2 subnets. You are basically subnetting the subnet. This makes the network more efficient, allows more hosts and room for future growth. I can provide you with some notes on VLSM if you are interested. It's pretty easy it doesn't take long to get a handle on it.

lantech

Actually I think you're talking about route summarization rather than VSLM.

I was going to try and explain it to you but looking at my post I was afraid I might confuse you even more. Maybe someone can come along and explain it to you better than I could.

Prasad Satav

kweevuss wrote: »

Ok so I think it might have just clicked, I basically need a subnet that would have room from the address 172.16.32.0-172.16.128.0? And thats the whole point and to obviously find one that doesn't waste a ton of IPs

That is right.!
you might want to watch CBT nuggets video for OSPF, wherein Jeremy Cioara has has nicely explained the concept of route summarization, Because the question looks like more of a route summarization concept than VLSM to me too..

kweevuss

Ok Sounds good, I'm still kinda confused on the subject now that even one says that. Thats one topic I'm going to try to look at better next time around

MickQ

Route summarisation is where you take a load of subnets (preferably contiguous) and advertise one subnet which contains them all.

e.g.
10.0.0.0 /24
10.0.1.0 /24

= 10.0.0.0 /23
(Please look up the why it's a /23 elsewhere if you don't already know)

VLSM - think of its name; Variable Length Subnet Masking
This is what allows us to use classless subnets.
The summarising I did above is just one nifty part of VLSM.

If the route is being advertised across a network class boundary (e.g. 172.16.0.0 to 10.0.0.0), then some routing protocols (e.g. EIGRP) will advertise the classful route. This can be problematic if you have split your classful addressing with a different classful network inbetween. Think about it. The 10.0.0.0 router has 172.16.0.0 /16 on two interfaces. Which is the right one to send the packet out?



If you want to do well in the CCNA, you'll need to be very good at subnet calculations, and VLSM is a part of that.

kweevuss

MickQ wrote: »

Route summarisation is where you take a load of subnets (preferably contiguous) and advertise one subnet which contains them all.

e.g.
10.0.0.0 /24
10.0.1.0 /24

= 10.0.0.0 /23
(Please look up the why it's a /23 elsewhere if you don't already know)

VLSM - think of its name; Variable Length Subnet Masking
This is what allows us to use classless subnets.
The summarising I did above is just one nifty part of VLSM.

If the route is being advertised across a network class boundary (e.g. 172.16.0.0 to 10.0.0.0), then some routing protocols (e.g. EIGRP) will advertise the classful route. This can be problematic if you have split your classful addressing with a different classful network inbetween. Think about it. The 10.0.0.0 router has 172.16.0.0 /16 on two interfaces. Which is the right one to send the packet out?



If you want to do well in the CCNA, you'll need to be very good at subnet calculations, and VLSM is a part of that.

I do understand about using classful and classless protocols, and having different subnets over a network. I think my major issue was that I was interchanging the words and I didn't realize that they are two different things.

Thanks for the help! This is really going to help me.

I have trouble seeing that picture though, its small and I can't seem to enlarge it to really read it.

MickQ

Good to hear that you're understanding the differences. Remember, it's more important to learn the why behind things working, the how then follows that.

Sorry about the pic. It was one of the more accurate ones I could find on the net without having to make one up. Basically, 192.168.2.0 /26 and 192.168.2.128 /26 on either side of a router, with 10.0.0.0 /30 and 10.0.0.8 /30 inbetween.