Comments on: Classful vs Classless vs CIDR vs FLSM vs VLSM

By: Sam

Sam — Sat, 24 Jan 2026 00:02:45 +0000

I feel like this was the first time VLSM really clicked in how it relates to and differs from Classful and CIDR notations. Thank you so much for making this!

By: Hil

Hil — Fri, 12 Jan 2024 07:32:50 +0000

OMG, this is brilliant! I’ve read a lot of resources to get a better understanding of CIDR & VLSM and always ended up confused. But when I read through your article, I can understand it easily. Thank you soo much 🙂

By: Steve Baker

Steve Baker — Tue, 08 Nov 2022 16:33:51 +0000

I would argue that FLSM (subnetting) is NOT legacy. And in fact is still used to aggregate networks together, that can then be distributed to regions for efficient route aggregation. Once the subnet is distributed to a region, we can then fall back to using VLSM to distribute that subnet out to meet our needs in that particular region. Let me explain. Let’s say you start with a 10.0.0.0/8 network for a large enterprise. We might first FLSM (subnet) this by geographical region. For argument sake, we will divide the ‘world’ into four segments. So we might FLSM our original 10.0.0.0/8 into four networks of /10 each. (Subnet A 10.0.0.0 /10, Subnet B 10.64.0.0 /10, Subnet C 10.128.0.0 /10 and finally Subnet D 10.192.0.0 /10) Subnet A might go to North America. We then use VLSM to segment this supernet network according to our needs within that region. Subnet B might go to the EU, then VLSM to split it out according to our needs in that particular region. So on and so forth… Point is, we still to this day utilize both subnetting and VLSM, they are just used for different purposes. Subnetting is more for route aggregation and ensuring networks remain contiguous within a particular region. This facilitates the ability to keep route tables small, leading to efficient and quick routing from region to region.

By: Ed Harmoush

Ed Harmoush — Fri, 14 Oct 2022 23:58:23 +0000

In reply to Oren. Thank you, Oren.

By: Oren

Oren — Thu, 28 Jul 2022 09:53:05 +0000

Really enjoyed the article. Topic well explained 🙂

By: Azza

Azza — Tue, 12 Apr 2022 00:21:37 +0000

Thank you

By: Ed Harmoush

Ed Harmoush — Tue, 29 Mar 2022 17:05:31 +0000

In reply to Martin D..

You can actually use a /31 on point to point links =)

https://datatracker.ietf.org/doc/html/rfc3021

I also mention it in my VLSM explained video.

By: Ed Harmoush

Ed Harmoush — Tue, 29 Mar 2022 17:04:24 +0000

In reply to Scott. Great point. I had that in my old blog theme. But it wasn't enabled in the new theme. I'll dig through the settings to re-enable it. Glad you enjoyed the article!

By: Martin D.

Martin D. — Sun, 13 Mar 2022 06:08:58 +0000

many thnaks! just a remark on last VLSM section two transport networks should be /30 not /31

By: Scott

Scott — Sat, 05 Feb 2022 01:44:34 +0000

This is a great article! I would suggest putting your name at the top and the date you posted this. I am using this as a reference to a paper I am writing for college. I was able to find your name by going to you about tab. I don’t think most people will do that. If you put both at the top of the page your work could be cited easier. Just an idea. Anyhow, very detailed with some great examples!