traditional question CIDR vs VLSM

hi all

sorry I did not understand it well,

10.0.0.0 255.255.248.0

is this VLSM or CIDR

how can we differnciate between them ?

Comments

  • Hi,

    Here a link explain CIDR vs VLSM:

    https://learningnetwork.cisco.com/thread/35267

    "

    The both deal with non-boundary binary...

    VLSM was designed for subnetting (making smaller).  CIDR was designed for supernetting/aggregating (making larger).

    "

    Thanks,

  • I have read this before but did not understand it,

  • Actually this represents both the concepts, because it can fulfill the host requirement using 2^11 and it also has classless concept. The first octect "10" belongs to Class A but its subnet mask doesn't belong to Class A default, So, its classless too. So bottom line is, based on the host requirement we can define variable prefix which is known as VLSM and defining address in classless way is CIDR.

    Hope this helps!

  • I have read this before but did not understand it,

     

    OKay,

    Let me try to explain you these 2 notions.

    VLSM ( supernetting, divide, a lot is better)

    Let  say you've a /24 network (for examlpe 172.16.1.0/24).

    In this network you've up 253 users in the same broadcast (.0 is for the nework and .255 is the broadcast).

    The notion of broadcast means the same domain of collisions. What can affect your network performance. So can divide this network into smaller broadcast domain.

    OR you want to subdivise this network to have for example subnet for DATA, for MULTIMEDIA, for ADMIN USERS.

    Then come in play VLSM which help you perform this need. For example you can subdivise  172.16.1.0/24 into small network each network in :

    - 172.16.1.0-15/28 - For your DATA ( 14 servers in this)

    - 172.16.1.16-31/28 - for your MULTIMEDIA

    - 172.16.1.32-47/28- for your  ADMIN Users

    and SO on

    CIDR ( Summarisation, aggregation, small is better).

    with CIDR you make the contrary the other way.

    Let say in your router you a lot of networks this consumme a lot of CPU and this result this bad performance.

    So CIDR come into play and help you summarize, aggregate a lot of networks to a single subnet.

    For example 172.16.1.0/24 can be simply summarized, aggregate to 172.16.0.0/16.

    Don't hesitate if it's clear and if more explanation is needed.

    We all learn from your answers/questions.

    Thanks,

     

     

     

     

  • My understanding was that CIDR dealt with breaking down traditional boundaries into non-traditional boundaries by using netmasks.  Think of taking a "Class A" network and assigning a single /16 out of it.  Hence "CIDR Notation."  To this day, you'll hear even very smart folks referring to any arbitrary /24 as a "class c" - terminology dies hard.

    I understood VLSM to be more about breaking an established bitmask down into smaller networks by adding to the network bits and subtracting from the host bits.  Meaning, to take a /24 and mask it as a /26, with the result being you now have 4 networks with 64 IPs each.

    I think at this point, you'll mostly hear about CIDR when talking about CIDR notation and RIR allocations, and mostly hear about VLSM when talking about IGPs.

  • Hi pieces_of_eight,

    You're right let enhance the Explanation:

    Traditional boundaries are subdivided by classes A, B, C, D, E:

    - A = 0 to 127.255.255.0 /8 - default mask   255.0.0.0

    - B = 128.0.0.0 to 191.255.255.255 /16 - default mask   255.255.0.0

    - C = 192.0.0.0 to 223.255.255.255 /24  default mask  255.255.255.0

    - D = 224.0.0.0 to 239.255.255.255 /?  default mask unspecified (has to begin with 224 to 239) => for multicast application

    - E = 240.0.0.0 255.255.255.255 - reserved for reacherches Lab -

    This classification remain true today.

    But let say that CIDR ( classless Interdomain Routing) permit us to go further and have for example more than default mask 255.0.0.0 in class A.

    I remimber that in the router we had to configure ip classless command to have CIDR so not to limit to the default class mask in each class.

    So let correct:

    CIDR ( classless Interdomain Routing) permit us to go further and have for  more than default mask 255.0.0.0 (/8) in class A for example 255.255.255.0 (/24) and so on.

    VLSM ( supernetting, divide, a lot is better)

    Let  say you've a /24 network (for examlpe 172.16.1.0/24).

    In this network you've up 253 users in the same broadcast (.0 is for the nework and .255 is the broadcast).

    The notion of broadcast means the same domain of collisions. What can affect your network performance. So can divide this network into smaller broadcast domain.

    OR you want to subdivise this network to have for example subnet for DATA, for MULTIMEDIA, for ADMIN USERS.

    Then come in play VLSM which help you perform this need. For example you can subdivise  172.16.1.0/24 into small network each network in :

    - 172.16.1.0-15/28 - For your DATA ( 14 servers in this)

    - 172.16.1.16-31/28 - for your MULTIMEDIA

    - 172.16.1.32-47/28- for your  ADMIN Users

    and So on

     * Summarisation, aggregation, small is better.

     

    Let say in your router you a lot of networks this consumme a lot of CPU and this result this bad performance.

    So summarization come into play and help you summarize, aggregate a lot of networks to a single subnet.

    For example 172.16.1.0/24 can be simply summarized, aggregate to 172.16.0.0/16.

    Don't hesitate if itsn't clear and if more explanation is needed.

    We all learn from your answers/questions.

     

    Thanks,

     

     

     

     


  • some minor correction in my post.

  • So in summary,

    CIDR => Not limit to the default class mask in each class.

    After that we can use VLSM to more size our network to  have small broadcast domain.

    CIDR and VLSM are interdependant. Without CIDR no VLSM.

  • hi all

    sorry I did not understand it well,

    10.0.0.0 255.255.248.0

    is this VLSM or CIDR

    how can we differnciate between them ?

    Hope that things are clear now with the explanations provided by community.

    With your example 10.0.0.0 255.255.248.0 without CIDR (Classless InterDomain Routing) it will be by default 10.0.0.0 255.0.0.0 and no more.  And with VLSM ( Variable Length Subnet Masking) we can get 10.0.0.0 255.255.248.0 and more.

    Thanks,

     

  • peetypeety ✭✭✭

    sorry I did not understand it well,

    10.0.0.0 255.255.248.0

    is this VLSM or CIDR, how can we differnciate between them ?

    This is at least CIDR ("classless" interdomain routing), because you're not limiting yourself to the old-school classful routing.  Classful routing rules would dictate that 10.0.0.0 must have a mask of 255.0.0.0, also known as a /8.

    Variable-length subnet masking refers to a network made up of subnets that aren't all the same length.  Old-school subnetting rules dictated that when you got a "network", you would choose a subnet mask size, and use that mask size consistently across your entire network.  I know of a university that has a Class B (a true class B: something in the 128-191 first octet range, and they had 65,536 addresses or a /16), and back when I was a student, they used /24 as their standard subnet mask size.  For the administrative LAN, they put multiple secondary addresses on the router port in order to make room for ~750 computers.  Nowadays with VLSM, they'd change that subnet to a /22 and make room for ~1000 computers, or perhaps make specific subnets for each department/region/hallway/etc., and size the subnet based on the number of devices and/or ports.

    We don't know if your example is VLSM, because we don't know the subnet mask of any other subnets you're using.  If all of your subnets are /17, it's not VLSM.

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