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CIDR improves the allocation of IP addresses. VLSM allows to define a subnet as we wish. Can CIDR used without using VLSM?

Rumesh Madhusanka
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3 Answers3

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VLSM from the classful era (RFC 950) predates CIDR (RFC 1518) but essentially they mean the same thing.

Strictly speaking, VLSM allows subnetting a classful network. With CIDR, there are no classes and everything depends on the prefix length.

Community
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Zac67
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  • If VLSM allow to fragment a classfull network, does that mean that before VLSM, an enterprise that want ten network had to request ten class A/B/C subnets? Or at that time they can order a single /8 and only subnet it in /16 or /24 ? – Nakrule Jan 21 '20 at 13:31
  • @Nakrule Classful networks ceased to exist in the mid 1990s. Honestly, I don't remember how those things were handled back then... – Zac67 Jan 21 '20 at 18:22
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To understand CIDR and VLSM, you'll want to also understand their "opposite" terms: Classful assignments and FLSM:

  • Classful and CIDR — these have to do with the size of networks as they are assigned from IANA.
  • FLSM and VLSM — these have to do with how you allocate your IP space within your networks.

To define each term individually:

  • Classful addressing is IANA/RIRs assigning IP space from Class A, B, or C blocks (legacy).

  • Classless or CIDR is IANA/RIRs assigning IP space in any size block, as required (modern standard).

  • FLSM, or Fixed Length Subnet Masking, mandates that every IP subnet within your deployment be the same size (legacy).

  • VLSM, or Variable Length Subnet Masking, allows any IP subnet within your deployment to be any size (modern standard).

More details here:

https://www.practicalnetworking.net/stand-alone/classful-cidr-flsm-vlsm/

(Disclaimer, the links in this post are to articles that I wrote. There are no ads. I make no revenue from visiting this site. I am providing the links purely to assist the reader.)

Eddie
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In CIDR , an IP network is represented by a prefix, which is an IP address and some indication of the length of the mask. Length means the number of left-most contiguous mask bits that are set to one. So network 172.16.0.0 255.255.0.0 can be represented as 172.16.0.0/16. CIDR also depicts a more hierarchical Internet architecture, where each domain takes its IP addresses from a higher level. This allows for the summarization of the domains to be done at the higher level. For example, if an ISP owns network 172.16.0.0/16, then the ISP can offer 172.16.1.0/24, 172.16.2.0/24, and so on to customers. Yet, when advertising to other providers, the ISP only needs to advertise 172.16.0.0/16.

Variable Length Subnet Masks (VLSM) allows you to use different masks for each subnet, thereby using address space efficiently.

VLSM Example

Given the same network and requirements as in Sample Exercise to develop a subnetting scheme with the use of VLSM, given:

netA: must support 14 hosts

netB: must support 28 hosts

netC: must support 2 hosts

netD: must support 7 hosts

netE: must support 28 host

so the difference is CIDR is supernetting while VLSM is explained better with the following:

204.15.5.0 can be broken down into 5 different networks given the example requirement above into:

netB: 204.15.5.0/27 host address range 1 to 30

netE: 204.15.5.32/27 host address range 33 to 62

netA: 204.15.5.64/28 host address range 65 to 78

netD: 204.15.5.80/28 host address range 81 to 94

netC: 204.15.5.96/30 host address range 97 to 98

guidev
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