Vlsm

hrobins

Hey guys, I have been lurking in here for a while, while trying to study for the CCNA test. I have a question about VLSM and trying to find the answer. I have been using Todd Lamie's book on this subject but still trying to get a grasp on this concept. What I am curious about is how you would use VLSM on Class A and Class B ip addresses? Thanks.


Howard Robinson

RouteMyPacket

hrobins wrote: »

Hey guys, I have been lurking in here for a while, while trying to study for the CCNA test. I have a question about VLSM and trying to find the answer. I have been using Todd Lamie's book on this subject but still trying to get a grasp on this concept. What I am curious about is how you would use VLSM on Class A and Class B ip addresses? Thanks.


Howard Robinson

Not sure why there is even a focus on "Class A, Class B" etc. That's dead and gone...CIDR is alive and well but as far as how VLSM affects a typical Class A....no different than anything else. All you are doing is taking a specific address space and subnetting it into pieces. It makes absolutely no difference what "class" it is.

What is the difference between these three address spaces?

10.10.0.0/24
192.168.0.0/24
172.31.0.0/24

All have the exact same amount of addresses, "but the 10 one looks like a Class A address"..that is irrelevant as we use CIDR. So say we had the following

Site A - 100 Hosts

Site B - 50 Hosts

Site C - 40 Hosts

Site D - 35 Hosts

How can we use VLSM to provide these sites with address space?

Ok, let's use 10.10.0.0/24

Site A needs the most hosts so we start there (always start largest to smallest), their space will be 10.10.0.0/25 giving them the following

10.10.0.0 - 10.10.0.127 which equals a 10.10.0.0/25

Site B needs 50, ok so we start from 10.10.0.128 so let's give them a 10.10.0.128/26 giving them the following range

10.10.0.128 - 10.10.0.191 which equals a 10.10.0.128/26

Site C needs 40, so let's start from 10.10.0.192 so let's give them a 10.10.0.192/26 giving them the following range

10.10.0.192 - 10.10.0.255 which equals a 10.10.0.192/26

We are completely out of space and can no longer subnet from the original /24..hmm we should have used something bigger eh? Say maybe a /23 perhaps?

Start with a /23 and follow the exact process above, biggest to smallest. Notice you can't pick and choose the exact amount of addresses, if a site needs 50 IP's, there is no means of assigning that exact amount which is why we provide them with a /26 which has 64 addresses..we have room for growth which is a huge best practice.

Site needs 100? We gave it a /25 which is 128 addresses...again room for growth and we don't have any other choice

/32 1 address
/31 2 addresses
/30 4
/29 8
/28 16
/27 32
/26 64
/25 128
/24 256
/23 512
/22 1024
/21 2048
/20 4096
/19 8192
/18 16384
/17 32768
/16 65536

Hope that helps and also better yet hope my subnetting if right off my head : )

Subnetting is not some monster people make it to be, it's a basic thing and once you grasp it, you don't think of it anymore you just do it.

--chris--

RouteMyPacket wrote: »

Not sure why there is even a focus on "Class A, Class B" etc. That's dead and gone...CIDR is alive and well but as far as how VLSM affects a typical Class A....no different than anything else. All you are doing is taking a specific address space and subnetting it into pieces. It makes absolutely no difference what "class" it is.

What is the difference between these three address spaces?

10.10.0.0/24
192.168.0.0/24
172.31.0.0/24

All have the exact same amount of addresses, "but the 10 one looks like a Class A address"..that is irrelevant as we use CIDR. So say we had the following

Site A - 100 Hosts

Site B - 50 Hosts

Site C - 40 Hosts

Site D - 35 Hosts

How can we use VLSM to provide these sites with address space?

Ok, let's use 10.10.0.0/24

Site A needs the most hosts so we start there (always start largest to smallest), their space will be 10.10.0.0/25 giving them the following

10.10.0.0 - 10.10.0.127 which equals a 10.10.0.0/25

Site B needs 50, ok so we start from 10.10.0.128 so let's give them a 10.10.0.128/26 giving them the following range

10.10.0.128 - 10.10.0.191 which equals a 10.10.0.128/26

Site C needs 40, so let's start from 10.10.0.192 so let's give them a 10.10.0.192/26 giving them the following range

10.10.0.192 - 10.10.0.255 which equals a 10.10.0.192/26

We are completely out of space and can no longer subnet from the original /24..hmm we should have used something bigger eh? Say maybe a /23 perhaps?

Start with a /23 and follow the exact process above, biggest to smallest. Notice you can't pick and choose the exact amount of addresses, if a site needs 50 IP's, there is no means of assigning that exact amount which is why we provide them with a /26 which has 64 addresses..we have room for growth which is a huge best practice.

Site needs 100? We gave it a /25 which is 128 addresses...again room for growth and we don't have any other choice

/32 1 address
/31 2 addresses
/30 4
/29 8
/28 16
/27 32
/26 64
/25 128
/24 256
/23 512
/22 1024
/21 2048
/20 4096
/19 8192
/18 16384
/17 32768
/16 65536

Hope that helps and also better yet hope my subnetting if right off my head : )

Subnetting is not some monster people make it to be, it's a basic thing and once you grasp it, you don't think of it anymore you just do it.

Excellent break down on VLSM. @ OP, study this until you get VLSM.

Deathmage

RouteMyPacket wrote: »

Not sure why there is even a focus on "Class A, Class B" etc. That's dead and gone...CIDR is alive and well but as far as how VLSM affects a typical Class A....no different than anything else. All you are doing is taking a specific address space and subnetting it into pieces. It makes absolutely no difference what "class" it is.

What is the difference between these three address spaces?

10.10.0.0/24
192.168.0.0/24
172.31.0.0/24

All have the exact same amount of addresses, "but the 10 one looks like a Class A address"..that is irrelevant as we use CIDR. So say we had the following

Site A - 100 Hosts

Site B - 50 Hosts

Site C - 40 Hosts

Site D - 35 Hosts

How can we use VLSM to provide these sites with address space?

Ok, let's use 10.10.0.0/24

Site A needs the most hosts so we start there (always start largest to smallest), their space will be 10.10.0.0/25 giving them the following

10.10.0.0 - 10.10.0.127 which equals a 10.10.0.0/25

Site B needs 50, ok so we start from 10.10.0.128 so let's give them a 10.10.0.128/26 giving them the following range

10.10.0.128 - 10.10.0.191 which equals a 10.10.0.128/26

Site C needs 40, so let's start from 10.10.0.192 so let's give them a 10.10.0.192/26 giving them the following range

10.10.0.192 - 10.10.0.255 which equals a 10.10.0.192/26

We are completely out of space and can no longer subnet from the original /24..hmm we should have used something bigger eh? Say maybe a /23 perhaps?

Start with a /23 and follow the exact process above, biggest to smallest. Notice you can't pick and choose the exact amount of addresses, if a site needs 50 IP's, there is no means of assigning that exact amount which is why we provide them with a /26 which has 64 addresses..we have room for growth which is a huge best practice.

Site needs 100? We gave it a /25 which is 128 addresses...again room for growth and we don't have any other choice

/32 1 address
/31 2 addresses
/30 4
/29 8
/28 16
/27 32
/26 64
/25 128
/24 256
/23 512
/22 1024
/21 2048
/20 4096
/19 8192
/18 16384
/17 32768
/16 65536

Hope that helps and also better yet hope my subnetting if right off my head : )

Subnetting is not some monster people make it to be, it's a basic thing and once you grasp it, you don't think of it anymore you just do it.

this is slowly starting to 'click' in my brain....

coralreefguy

Don't stop subnetting. Do it until your eyes bleed. Then do it some more. You're getting there!