How to add VLSM to an existing VLSM

Fangerino · ‎08-13-2023

Hi, Can someone teach me how to extend my VLSM (I'm not sure is it the correct way to say it).

Please refer to the attached, lets say my company decided to have 1 more dept and I was tasked to give them a network how should i go about it from /30 ?

Flavio Miranda · ‎08-13-2023

Hi @Fangerino

You dont go anywhere with a /30 network.

A /30 network has only 2 available address and thats why we use it for point to point links.

If you were given a network like /24 which gives 254 address you can break in two networks with 126 address by changing the mask from 255.255.255.0 to 255.255.255.128.

Or you can break in 4 network of 62 address by changing the mask to 255.255.255.192.

So on so forth until you get to the /30 which is 255.255.255.252

Joseph W. Doherty · ‎08-13-2023

I'm responding from my phone, so cannot see attachment, but from a /30? As a /30 only has 4 addresses, two of which are host addresses, not going to get far splitting it. Plus, there are "conditions" for using a /31 or /32.

Later today, should be able to view your attachment, and then, perhaps, can suggest IP allocation techniques.

Oh, also possibly this isn't the best topic forum for your question. (Again, I'll need to see your attachment.)

Joseph W. Doherty · ‎08-13-2023

Ah. I'm on my PC and I can now view your attachment.

As I and @Flavio Miranda noted earlier, you normally don't subdivide a /30 (although /31s might be supported, but that would only be useful to support four p2p links using the same address space now supporting the two p2p links).

From your attachment's table, I wonder if the rest of the 10.0.3.0/24 address block is unused and/or what else might be available out of 10.0.0.0/8. If the rest of 10.0.3.0/24 is available, you can carve out a /25 and/or other/smaller networks.

Fangerino · ‎08-14-2023

Hi, I was given a network address of 10.0.0.0/16 and have divided according to previous image from what I read I need to create a new network address? Because by breaking do I need to change my whole VLSM? Because I don't know anything about adding new VLSM without changing the whole network. My assignment state "One year down the road your company decides to add a new dept without changing the network."

pieterh · ‎08-14-2023

other responses already explained the /30 subnet is only for the point-to-point connection between your inside network and the outside

in your sheet you have a separate address range for each floor
VLSM means variable length subnet mask , here the /24, /25 etc. comes in as the "length" of the subnet mask

if you need to add an extra floor you can add for example 10.0.4.0/24 for the new floor floor

if you need to expand the address range on an existing floor
you can change from /25 to /24 netmask increasing the address range from 127 to 256 addresses
this cannot be done on floor-4 because this aready is /24
if you need to expand floor-4 adres space, this needs an extra step

Joseph W. Doherty · ‎08-14-2023

@pieterh wrote:

in your sheet you have a separate address range for each floor

if you need to add an extra floor you can add for example 10.0.4.0/24 for the new floor floor

You could, but it doesn't have to be a /24, and if smaller, like perhaps like another /25, that could be allocated as 10.0.3.128/25, correct?

@pieterh wrote:

if you need to expand the address range on an existing floor
you can change from /25 to /24 netmask increasing the address range from 127 to 256 addresses
this cannot be done on floor-4 because this aready is /24
if you need to expand floor-4 adres space, this needs an extra step

Hmm, changing from an existing /25 to a /24 would require jumping to a completely new /24 address block, correct? I.e. more than just a netmask change. (This because the way the /25s are allocated, they cannot be expanded into /24s.)

Regarding the existing /24, unclear how that differs from expanding the other floor networks. It too, with the current allocations, cannot be expanded to be a /23, correct? So, might you elaborate why expansion cannot be done for floor-4, using the /24, versus expansion of any of the other floor networks? Even floor 1's /28 cannot be expanded because of the two /30s.

pieterh · ‎08-14-2023

Hello @Joseph W. Doherty

I overlooked the /25 networks are consecutive, so you're right my suggestion to expand the /25 network to a /24 is not possible
with my statement about the /24 I did mean the same , it cannot be expanded with 10.0.1.0/24 to a /23

Joseph W. Doherty · ‎08-14-2023

If your network allocation is 10.0.0.0/16, you have 10.0.0.0 through 10.0.255.255 to allocate from.

For a new department, unlikely you'll lack for address space, but how to add such a department might be very, very easy (new department hosts added to already defined subnets) to very, very complex (the latter if you do need to change your whole VLSM scheme), to a whole range between those extremes.

What's your case? Insufficient information to say.

However, most likely, if you need to allocate additional address space, you would pick up where you left off.

You're likely thinking, but I've allocated from large to small. Yup! Do you understand why that's suggested?

Joseph W. Doherty · ‎08-14-2023

My assignment state "One year down the road your company decides to add a new dept without changing the network."

BTW, adding new networks IS changing the network. lf this means not to change the now existing network allocations, again, as you have a /16 to work with, and haven't fully used a /22, very likely you have ample available address space.

If you don't understand how to manage address space, do what you did before starting with 10.0.255.255 but allocating larger to smaller moving down. If your new address blocks have nothing larger than a /22, you can start with 10.0.4.0, and work much as before.

Neither of the above may be ideal but then neither is the (usually recommended) approach you've already used if network allocation changes arise later.

pieterh · ‎08-14-2023

My assignment state "One year down the road your company decides to add a new dept without changing the network."

if you look at it that way....
this network subnetting is based on floors, not on departments
so you can put the new department on any floor that has enough address space
- floor 4 has the largest nework available
- floor 1 the smallest
You can even split a department over multiple floors. without changing anything
(regarding the network, not regarding organization of course)

Joseph W. Doherty · ‎08-14-2023

@pieterh wrote:

My assignment state "One year down the road your company decides to add a new dept without changing the network."

if you look at it that way....
this network subnetting is based on floors, not on departments
so you can put the new department on any floor that has enough address space
- floor 4 has the largest nework available
- floor 1 the smallest
You can even split a department over multiple floors. without changing anything
(regarding the network, not regarding organization of course)

Well, that's possible, but I think it unlikely for a subnetting assignment. Plus, in the real-world, my experience has been to support floor layouts, which might change, not we assign host locations where we have an excess of address space.

pieterh · ‎08-14-2023

@Fangerino
can you please explain more about the assignment? where does the extra dept need to be situated?
an existing floor?, an extra floor?, an extra building/site ?

or did the company mean you leave some space between the subnets instead of all consecutive?
like 10.0.10.0/24 and 10.0.20.0/24, .... for the floors
leaving the subnets between 10.0.1.0/24 until 10.0.9.0/24 free for future expansion?

Fangerino · ‎08-15-2023

Hi All, Thanks for the help, I just want to know if is possible to extend a not, I have attached a working packet tracer with the VLSM. As for whether is for existing floor or extra floor or whatsoever does not matter I was thinking if is actually possible to add on new subnet to existing but if there's no way then I guess I will have to wait for the lecturer review/answer

Joseph W. Doherty · ‎08-15-2023

". . . I just want to know if is possible to extend a not . . ."

Depends what you mean by extend.

In a real-world situation, you would want to know how many of these new department hosts there will be, and where.

You'll also need to know, whether you need some form of "security" traffic filtering between these new hosts and other parts of your network.

Assuming, there's no security issues, as you have various amounts of free host IPs, in different networks, again, depending on where these new department hosts will be, you might only need to connect them into an existing network.

If these new department hosts will be somewhere completely new, you'll need to allocate new networks, much (but possibly not exactly) as you did before.

If these new departments hosts will be at locations served by an existing subnet, without sufficient number of available hosts, you'll need to again define a new network for that location. However, again depending on network security issues, you might size the new network to handle just the new department's hosts, or the number of new department hosts that are in excess of what the current location's network has available to use.

Again, actual new network allocating can be done much like before.

In one of my earlier posts, I asked do you understand why the initial allocations were done as they were. If so, it's not too difficult to understand how to "ideally" allocate additional networks. If you don't understand, why VLSM was allocated as it initially was, you need to understand that first. If you don't understand, we can try to help you to understand.