Difference between CIDR and VLSM

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Hello!
I didn't get quite well what is the exact difference between CIDR and VLSM. It kinda same the same in the show; can you please explain a little further?
Thanks

@Jorge-Sosa,

VLSM is technically when you use multiple subnet masks within your network to conserve address space. For example: Your LAN may have need of 119 IP addresses for SALES, I need 4 networks that has 8, 10, 12, 14 devices, respectively and then the Reception needs 6 IP addresses and 3 point to point connections to the branch offices.
VLSM allows me to do this for my 192.168.1.0/24 that I've been assigned:

SALES:  192.168.1.0/25

ADMIN: 192.168.1.128/28
MKTG: 192.168.1.144/28
IT: 192.168.1.160/28
DEV: 192.168.1.176/28

RECPT: 192.168.1.192/29

LAN to MIA: 192.168.1.200/30
LAN to LA: 192.168.1.204/30
LAN to NY: 192.168.1.208/30

CIDR helps us to clearly delineate the addresses that use by setting up a prefix notation instead of using a subnet mask to make it easier to use VLSM.

So one is a method (VLSM) of doing, the other is a method of reporting what is done.

Wow Ronnie I've never heard it explain like that before, NICE!. I always just assumed one was old school and CIDR was new school. Most of the new network equipment and devices seem to use CIDR notation instead of subnet mask for example the cisco meraki stuff and the samsung galaxy tabs I have both use CIDR.

Found some more info for further clarification:

• CIDR is the chunk of meat the provider gives you, VLSM is how you serve it to your guests
• CIDR is a concept applied more at the ISP/Provider level. VLSM is a concept applied more at interior routing within an enterprise; a mechanism used by the recipient of a CIDR block.
• CIDR allows super-netting for efficient advertisement and summarization of the domains 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; This helps in reducing the size of full Internet BGP table
• If the ISP / provider gives you a /24 CIDR block, you will likely use VLSM to subnet it into a smaller blocks

Source: https://supportforums.cisco.com/discussion/10547531/what-difference-between-cidr-and-vlsm

@Jorge-Sosa said in Difference between CIDR and VLSM:

Found some more info for further clarification:

• CIDR is the chunk of meat the provider gives you, VLSM is how you serve it to your guests
• CIDR is a concept applied more at the ISP/Provider level. VLSM is a concept applied more at interior routing within an enterprise; a mechanism used by the recipient of a CIDR block.
• CIDR allows super-netting for efficient advertisement and summarization of the domains 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; This helps in reducing the size of full Internet BGP table
• If the ISP / provider gives you a /24 CIDR block, you will likely use VLSM to subnet it into a smaller blocks

Source: https://supportforums.cisco.com/discussion/10547531/what-difference-between-cidr-and-vlsm

The original idea for it is when the ISP were allowed to issue out to customers a smaller size that the classful addressing scheme. It was easier for them to subdivide their massive pools of addresses they were responsible for leasing if they looked at their pool of a /18 to know they had this many addresses they could use and then VLSM from that point.

@Daniel-Espinal said in Difference between CIDR and VLSM:

Wow Ronnie I've never heard it explain like that before, NICE!. I always just assumed one was old school and CIDR was new school. Most of the new network equipment and devices seem to use CIDR notation instead of subnet mask for example the cisco meraki stuff and the samsung galaxy tabs I have both use CIDR.

No doubt, which would you rather type? 255.255.255.252 or /30 or 255.255.255.248 or /29 I could see why it's preferred. The irony behind this is that it requires you to understand the process of subnetting and the use of VLSM before you can really use the SIMPLER prefix-notation (e.g. /27 or /25) to indicate the number of bits that are your subnet mask. If don't have VLSM or basic subnetting down...then a 192.168.10.0 /27 is totally meaningless and useless to you.

Great, thanks again @Ronnie-Wong

@Ronnie-Wong Or maybe we will have a new generation of Network techs who just understand that /24 means 24 bit masks without ever having to have been taught the 255.255.255.0 notation.

Really if you think about it there is no need to labor over having to know that a /9 = 255.128.0.0 if you inherently understand that /9 equals a 9 bit mask.

@Daniel-Espinal,

I hate to say it outside of the realm of cisco certification and hiring mangers who want make candidates squirm, manually subnetting is considered an arcane witchery anyhow. Most network admins uses subnetting calculators instead and don't even try to learn subnetting anymore. I could be more crotchety and say, everyone should know it but the reality is that IPv6 is coming and subnetting will then truly be a lost art...like driving a manual gear shift vehicle. Great to know how but not many will know.