Vlsm subnetting out of order (gap)

sam

4/26/24, 6:15 PM

In VLSM we usually subnet in descending order from the network with highest number of hosts to the lowest one , but i was reading that "If you allocate subnets in any other order you will not be able to avoid leaving gaps between the subnets."

I tried with simple example and subnetted out of order to understand the meaning of the gap , but i couldn't understand the where the gap is ?

Can someone please explain this with an example ?

Regards

113

1 + 4

vlsm

Tom Yan

4/27/24, 4:58 AM

Well, for example if you need a /25 and /28 out of a /24, and you have the /25 at the second half of the /24, there will be a "gap" between the two subnets in the sense that their IP ranges are not contiguous to each other (even when the subnet ID and broadcast address are not considered).

Tom Yan

4/27/24, 5:00 AM

But then while it's an unavoidable gap, it's not a necessarily an unusable gap. You can fill more /28s (or even, a /26) into the gap when necessary. The gap can be fully filled, but it depends on what you want to fill into it. (Like, you certainly can't fill another /25 into it because the gap isn't wide enough.)

Tom Yan

4/27/24, 5:05 AM

If you don't understand why I said "second half", then you don't really understand how subnetting work yet.

sam

4/27/24, 3:03 PM

@TomYan thanks a lot

Score:0

Server

Peter Zhabin

4/26/24, 11:09 PM

Technically, there won't be "unavoidable" gaps, but convenience will certainly suffer.

Suppose you start with longest masks, i.e. you allocate first 192.168.1.0/28 (address range 0..15) and then you've decided that you need to allocate /26.

The closest range where you can start it would be 192.168.1.64/0, as /26 can only start at 0, 64, 128 and 192. This leaves you with an unused gap from .16 to .63, that you sure can use for three /28 or one /28 and one /27, or a bunch of smaller networks, but what if you don't have these smaller nets in existence?

Now if you do the reverse, and allocate /26 first, like 192.168.1.0/26, then you can allocate your /28 right away from 192.168.1.64/28 leaving no addresses left unused in between.

To be precise, allocating largest blocks first reduces the fragmentation of address space and increases convenience of managing it.

+ 1

sam

4/27/24, 3:06 PM

thanks a lot it makes sense now .

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Vlsm subnetting out of order (gap)

Post an answer

Most people don’t grasp that asking a lot of questions unlocks learning and improves interpersonal bonding. In Alison’s studies, for example, though people could accurately recall how many questions had been asked in their conversations, they didn’t intuit the link between questions and liking. Across four studies, in which participants were engaged in conversations themselves or read transcripts of others’ conversations, people tended not to realize that question asking would influence—or had influenced—the level of amity between the conversationalists.