Variable Length Subnet Masks Help.

Hey all,

I am currently studying for my CCNA, and am reading the SYbex book. I am reading the chapter on Subnetting, and have reached the topic of Variable Length Subnet Masks (VLSMs). I am really lost on this topic. Does anyone know of a good resource to learn about this? Better yet, has anyone ever had a test question on this topic?

Any help, greatly appreciated!

Thanks in advance.......

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Comments

viper75

VLSM's is a tough topic on the CCNA. I just finished my Semester 3 on the Cisco Academy Program and I myself had trouble on that area

. My final had a few questions on VLSM's. I can't remember off the top of my head what they were, but when I do remember I will post them up

. From what I hear from my Prof. and from the reading I've done Cisco wants you to know that topic very well. One of my weak areas along with Subnetting

viper75

Here...

1)Which interior routing protocols support VLSM? (Choose three.)

1-OSPF

2-RIP v1

3-RIP v2

4-EIGRP

5-BGP

6-IGRP

2)Which of the following problems does VLSM help to alleviate?

1-the shortage of IP addresses
2-the difficulty of assigning static IP addresses to hosts in large enterprises
3-the complexity of implementing advanced routing protocols such as OSPF and EIGRP
4-the shortage of network administrators qualified in the use of RIP v1 and IGRP

3) What does VLSM allow a network administrator to do?

1-utilize one subnet mask throughout an autonomous system
2-utilize multiple subnet masks in the same IP address space
3-utilize IGRP as the routing protocol in an entire autonomous system
4-utilize multiple routing protocols within an autonomous system

4) What is the purpose of the network command used in the configuration of the RIP routing protocol?

1-It specifies RIP v2 as the routing protocol.
2-It enables the use of VLSM.
3-It specifies the fastest path to the destination route.
4-It specifies which interfaces will exchange RIP routing updates.
5-It activates RIP for all routes that exist within the enterprise network.

Those were some questions on my assestment test and final exam. I didn't put the answers up because you might want to look them up, if you want me to put the answers up just let me know.

Smoothie

Thanks for your response. I'll look over VLSM's some more. There are so many areas that you need to cover when going for your CCNA. They don't make it easy.......

celticfc

Smoothie wrote:

Hey all,

I am currently studying for my CCNA, and am reading the SYbex book. I am reading the chapter on Subnetting, and have reached the topic of Variable Length Subnet Masks (VLSMs). I am really lost on this topic. Does anyone know of a good resource to learn about this? Better yet, has anyone ever had a test question on this topic?

Any help, greatly appreciated!

Thanks in advance.......

What exactly are you having trouble understanding? I would have to say that the section on VLSM's in the subnetting chapter is very brief as well, without understanding EIGRP and how it deals with VLSM's it doesnt make a lot of sense. You said youre reading the Sybex book, if its the newer 640-801 book, check out page 270 and read about EIGRP real quick, I guarantee that the VLSM information will make more sense.

Cheers

JSW77

just remember... all vlsm is is subnetting a subnet. in other words, if you have a pie that you cut up into equal size pieces, that is subnetting, and any classful protocol can do that. but if you then cut up a couple of those pieces again, that is vlsm, and you need a classless protocol to do this.

freebird

1,3,4
1
2
4
right?

joyblade

Where may i get the rest of the questions for modules? Since i am studying alone i do not have a chance to check my mistakes

kerberos

Smoothie

are you still having probs with VLSM? If you are then maybe I could be of some assistance. Which part are you shaky about?

Let me say abit about VLSM. But first lets start with classful and classless IP addresses. As you would know, classful addresses fall in the respective classes, A,B,C. that is they have a subnet of 255.0.0.0, 255.255.0.0 and 255.255.255.0 respectively. Now if you were supposedly an ISP and had to give out blocks of IP addresses to your customers to use on the internet, you will pretty soon run out of your allocation if you were to use classful addresses. The reason for this being that you will have to give out a class C for instance where only 3 or 4 addresses maybe required. Now thats what I call wasteful! What if instead of giving out the whole class C, you gave only as much as was needed? For instance if someone wanted to just make a point to point connection. In this case they would need 2 ip addr and thus that is all you should give. this will thus be a 255.255.255.252 mask. Now, this means that we are using classless IPs since these masks dont fall in the category of the normed classes A,B,C.

Now back to VLSMs. VLSM basically means that in your network instead of having all classful IP addresses, you have classless addresses also. Thats why it is called variable length subnet masks since instead of having just one class of IPs outright for all the machines, you have a mixture of ip address masks.

Hope this helps.