Is there any specific advantage that VLSM or FLSM has over the other when it comes to route summarization? I was of the opinion that neither had an advantage over the other when it came to route summarization, but I just wanted to confirm. It's easier for us (humans) to do subnetting via FLSM but a router doesn't care does it?

  • If you mean classful routing by FLSM, then that has been deprecated for over 20 years. All modern network equipment now uses VLSM. – Ron Maupin Sep 5 '16 at 17:29
  • Okay but that's due to the flexibility it provides us in dividing the subnet right? Is there any advantage when it comes to router and route summarization? – Danis Fermi Sep 5 '16 at 18:24
  • In the real world, what we deal with here, everybody uses VLSM. If anybody asks you about classful routing, that was deprecated over 20 years ago, and it is no longer used. – Ron Maupin Sep 5 '16 at 18:28

Danis, you have the right idea. To answer the question what is the diffrence ... FLSM can be thought of like address summarization that can't be taken apart. FLSM can be efficient at routing because it doesn't even need the whole IP to do its routing but it is an inefficient use of ip addresses since it reserves the whole block for 1 subnet only. The FLSM is usually a string of binary digits shown over the subnet number, telling the router which parts of the subnet number to look at. A binary "1" over a particular digit in the subnet number says "Pay attention to this digit." A "0" says "Ignore this digit." Allowing it to not focus on the whole IP are. Reference click here

VLSM summerization is efficient also when you do route summarization by combining subnet, but allows the addresses to be broken into classless (not forced to class A, B or C) subnets.

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