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Community Member

vlsm

Is this where we would subnet a say /26 down even further, can anyone give me an easy way of working these out ? I had an instructor who drew a table ?

8 REPLIES

Re: vlsm

vlsm - means that you'll have different subnet mask in your network for the same network. This will help you better use your IPs. as you can apply a /30 to your serial point-to-point links and /24 or others (/25, /26...) to your LAN for instance.

i.e. for class A x.x.x.x/8, you'd have /10, /12, /20 etc in different parts of your network.

Even the concept of classes (A,B,C...) is getting faded as its rare to use classfull routing protocols.

HTH,

Id appreciate if you rated this post.

Vlad

Community Member

Re: vlsm

Im still unsure on this, I thought it was say getting say 4 x /26 subnets out of a /24 ?

can you explan ?

thanks a million

Carl

Community Member

Re: vlsm

Yes. With FLSM (Fixed Length Subnet Masks), all of the subnets have the same mask. So, a /24 could be broken down into 2 /25's, or 4 /26's, or 8 /27's. The problem here is that if I only needed 2 IP addresses, I just wasted plenty of address space. With VLSM, I could have a /25, a /26 and 2 /27's, or I could have 3 /26's, a /27, a /28, a /29, and 2 /30's. It all depends on your requirements.

The way I use is to start with the major block you have, and work you way down. I'll use a /24 in this example. I like to think of this as the "pie method". The /24 is considered a whole pie. If I cut the pie in half, I have 2 halves, or 2 /25's. I could take one of the halves (/25's) and cut that in half, giving be fourths. At this point, I would have three slices of pie, 1 half and 2 fourths. Altogether, you still have the one pie, but just in different pieces to meet your needs. If you need a /27, just cut one of the /26's (fourths) in half. For a /28, cut the /27 in half.

This is just one of the many ways people use for VLSM. Ultimately, you have to find a way that works best for you.

HTH.

Eugene

Community Member

Re: vlsm

thats a good example, Can you give me an example wheras you include an ip address ?

Thanks

Carl

Community Member

Re: vlsm

Sure.

Take the network 192.168.0.0/24. Let's say I need one subnet of 100 users, one of 50 users, and one of 25 users (I am assuming that I am not expecting any growth).

Well, I need three subnets. The /24 will give me 254 hosts on one network. I can cut the /24 into two pieces of "pie", or 2 /25's. First, what will those subnets be? Well a /25 in binary is 11111111.11111111.11111111.10000000. If you look at where the last 1 fell, it lands on the 128 bit in the forth octet. So, this means that my subnets will jump by 128 in the forth octet.

My two /25's are:

192.168.0.0/25

192.168.0.128/25

Looking at the binary mask again, I have seven 0's at the end. The zeros represent the host portion of the mask. I can calculate the number of hosts/subnet with the following equation:

(2^n) - 2, where n = the number of 0's = 7

(2^7) - 2 = 128 - 2 = 126.

So, I could use 192.168.0.0/25 to satisfy the 100 user subnet requirement. I still need to satisfy the other two requirements.

The address block I have left is 192.168.0.128/25. I will cut this "piece" in half, into 2 /26's. The mask in binary is 11111111.11111111.11111111.11000000. If you look at where the last 1 fell, it lands on the 64 bit in the forth octet. So, this means that my subnets will jump by 64 in the forth octet.

My 2 /26's are:

192.168.0.128/26

182.168.0.192/26

Looking at the binary mask again, I have six 0's at the end; my hosts/subnet will be:

(2^n)-2, where n = 6

(2^6)-2 = 64 - 2 = 62.

That will satisfy the rest of the requirements. If I wanted to get more granular, I could have "cut" one of the /26’s into two /27's:

192.168.0.192/27

192.168.0.224/27

This would give me two subnets of 30 users, so I could use one subnet and have one to play with. In this example, I wasn't counting on any growth, so it wouldn't matter.

HTH,

Eugene

Community Member

Re: vlsm

Is there a tables of some sort you can draw for this to help you out, my last lecture , the instructor used a table to make it easy ?

thanks

Community Member

Re: vlsm

Hi there, So in basic is this just subnetting ? but all the subnetted addresses still belong to that network you were initially given ? i.e 192.168.1.0, anything where we subnet this down, would we just advertise the 192.168.1.0 network in eigrp and this would cover anything i have subnetted down too ?

Blue

Re: vlsm

use this link for some tables:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f33.shtml

basically, if the subnets are downStream of the interface you want eigrp to advertise them, then yes you can use a summary address such as 192.168.1.0 to cover all subnets defined under 192.168.1.0/24

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