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Can someone help explain VLSM for me.

Hello. I am looking for someone who actually understands VLSM to explain a few things for me. If this wasn't the correct forum for this post, please move it so that the correct people can answer it for me. I have a few major discrepancies which are making it very difficult to understand. When I first learned about VLSM, I understood is as simply a way to more efficiently use subnetted addresses by placing unused host addresses in another subnet. For example, if I took 192.168.0.0 I could subnet it as 192.168.0.0/26 and get 4 subnets with 62 usable hosts each. I then figured that at least on of these subnets would be used for a large group of hosts. The way I understood VLSM was that if I needed, say, 4 subnets with 30 hosts each, I could then split the 192.168.0.64/26 subnet into 192.168.0.64/27 and 192.168.0.96/27. At this point, I believed that I had used up this second original subnet, and would then have to go back and split up the 3rd original subnet. However, I have been told that I do not need to go back and take the rest of my subnets from the 3rd original, but instead, I can continue to subnet under the /27 mask all the way up to 192.168.0.224/27. I was told that after doing this, I could then go and use the 3rd subnet for something else, for example, subnetting it as 192.168.0.128/28 and 192.168.0.144/28. Which one of these methods is correct, or might they both be? The more detail you can add the better, as I will simply return and ask more questions if you don't. Thanks in advance!

3 REPLIES
VIP Purple

Re: Can someone help explain VLSM for me.

Hello,

first of all I think you won´t get a lot of responses if you aggressively threaten people. This forum is great and loaded with knowledgeable, helpful people, but you need to apply to certain standards, one of them being to stay polite :)

To answer your question:

192.168.0.0 --> can be subnetted into 4 subnets with a 26-bit mask --> correct

192.168.0.0/26 --> can be further subnetted into 4 /27 subnets --> correct

From here on, you can go and further subnet the /27 subnets into e.g. /28, /29, /30 subnets. I think what people refer to when they say that your subnet is used up is that you cannot go back and configure e.g. a /28 subnet and a /27 subnet out of the original 26 subnet, you have to ´stick´ with one mask because otherwise you have overlapping address spaces. You can easily test this by configuring to loopbacks on your router:

Router#conf t

Router(config)#interface loopback 1

Router(config-if)#ip address 192.168.1.1 255.255.255.192

!

exit

Router(config)#interface loopback 2

Router(config-if)#ip address 192.168.1.2 255.255.255.224

-->192.168.1.1 overlaps with Loopback1

Hope this helps.

Regards,

Georg

New Member

Re: Can someone help explain VLSM for me.

Thank you so much. I hope my original post did not come across as aggressive - if it did I'm sorry, It wasn't meant to sound that way. I will be more carefull in my wording in the future.

Just to make sure I have this correct, I will ask a few extra questions. If I were to take an address such as 192.168.0.0 and subnet it using VLSM, will I ever be able to get more than 254 usable host addresses out of it, or is VLSM simply a way to use as many of those hosts as you can and prevent them from going to waste.

My second question would be, can you EVER have two IP addresses that are the same, EXCEPT for the mask, or, are you limited to only using each host address once, but you can use different masks for different sub-subnets.

Finally, when I originally learned subnetting, I was told that to calculate the number of subnets you would be making, take 2^(# of bits for use in subnet). So, if I took the address 192.168.0.0 and subnetted it by borrowing 2 bits, I would have 192.168.0.0/26 and 192.168.0.64/26 and 192.168.0.128/26 and finally 192.168.0.192/26 - 4 subnets. So far, it worked, as 2^2 = 4. Now, this whole confusion started because a good networking buddy of mine said I could now subnet the 192.168.0.64/26 subnet by borrowing an additional bit. He claimed because I now had 3 bits in use for the subnet, I should be able to have 8 sub-subnets. I countered that we had not borrowed an additional 3 bits, but rather only 1 bit, and could only make 2 sub-subnets. Are either of these theories correct, or are we both stuck using the old set of rules we learned when we first started doing subnetting, and this no longer should apply.

Thank you in advance for any input you can provide!

Travis D. Cook - CCNA Student

New Member

Re: Can someone help explain VLSM for me.

"If I were to take an address such as 192.168.0.0 and subnet it using VLSM, will I ever be able to get more than 254 usable host addresses out of it, or is VLSM simply a way to use as many of those hosts as you can and prevent them from going to waste."

If you were to subnet this network with VLSM you will never get 254 hosts. When you use VLSM you create 'more' networks which each contain a network and a broadcast address out of this range, thus your amount of available host addresses will get less. VLSM allows you to optimise usage of your available addresses. If you only have say 5 hosts and you have a class C network you are wasting a lot of addresses, especially if they a public addresses.

"My second question would be, can you EVER have two IP addresses that are the same, EXCEPT for the mask, or, are you limited to only using each host address once, but you can use different masks for different sub-subnets."

My answer to this would be NO as the whole point of having a address is for it to be unique. If you have two hosts with the same address on the same or different networks how would the router(s) know which is the right one ??? BUT you did say EVER so I'm going to say that it can be done but then you have to use NAT (Network Address Translation), but you will learn all about NAT when you do your CCNP.

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