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Cidr & Vlsm

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  • Cidr & Vlsm

    Hi Guys
    Does anyone have very brief answer on what is difference betwween CIDR and VLSM
    concept ??

  • #2
    Re: Cidr & Vlsm

    Google is a good source...

    Network Consultant/Engineer
    Baltimore - Washington area and beyond


    • #3
      Re: Cidr & Vlsm

      CIDR means that you are ignoring the original classes (A, B, C) and that you can use whatever subnet mask you what with whatever range of IP addresses.

      VLSM just means that you can use more or less bits in the subnet mask to create smaller or larger subnets.

      For example, when talking about OSPF you could say that it is a classless routing protocol (supports CIDR) and it supports the use of VLSM.

      If you have more questions, post em' here!

      David Davis - Petri Forums Moderator & Video Training Author
      Train Signal - The Global Leader in IT Video Training - Free IT Training Products
      Personal Websites: &


      • #4
        Re: Cidr & Vlsm

        Thanks alot for your sharing David
        I will post new quest now
        wishing you all the best


        • #5
          Re: Cidr & Vlsm


          Routers use a form of IPv4 addressing called Classless Interdomain Routing (CIDR) that ignores class.

          CIDR is pronounced cider. In a classful system, a router determines the class of an address and then identifies the network and host octets based on that class. With CIDR, a router uses a bit mask to determine the network and host portions of an address, which are no longer restricted to using an entire octet.

          CIDR was first introduced in 1993 by RFC 1517, 1518, 1519, and 1520, and later deployed in 1994. CIDR dramatically improves the scalability and efficiency of IPv4 by providing the following:

          Replacement of classful addressing with a more flexible and less wasteful classless scheme.
          Enhanced route aggregation, also known as supernetting or summarization.
          Supernetting, which is the combination of contiguous network addresses into a new address defined by the subnet mask

          CIDR allows routers to aggregate, or summarize, routing information. It does this by using a bit mask instead of an address class to determine the network portion of an address. This shrinks the size of the routing tables used by the router. In other words, just one address and mask combination can represent the routes to multiple networks.
          Without CIDR and route aggregation, a router must maintain many individual entries for the Class B networks.

          The shaded columns in Figure identify the 16 bits that, based on the rules of class, represent the network number. Classful routers are forced to handle Class B networks using these 16 bits. Because the first 16 bits of each of these eight network numbers are unique, a classful router sees eight unique networks and must create a routing table entry for each. However, these eight networks do have common bits

          Variable-Length Subnet Mask (VLSM) allows an organization to use more than one subnet mask within the same network address space. Implementing VLSM is often referred to as subnetting a subnet and it can be used to maximize addressing efficiency.
          Consider the subnets created by borrowing three bits from the host portion of the Class C address,

          Unfortunately, there are only three subnets left for future growth and three point-to-point WAN links between the four sites remain to be addressed. If the three remaining subnets were assigned to the WAN links, the supply of IP addresses would be completely exhausted. This addressing scheme would also waste more than a third of the available address space.

          There are ways to avoid this kind of waste. Over the past 20 years, network engineers have developed three critical strategies for efficiently addressing point-to-point WAN links:

          Use VLSM
          Use private addressing (RFC 191
          Use IP unnumbered
          Private addresses and IP unnumbered are discussed in detail later in this module. This section focuses on VLSM. When VLSM is applied to an addressing problem, it breaks the address up into groups or subnets of various sizes. Large subnets are created for addressing LANs and very small subnets are created for WAN links and other special cases.

          A 30-bit mask is used to create subnets with two valid host addresses. This is the exact number needed for a point-to-point connection

          Routing Information Protocol version 1 (RIP v1) and Interior Gatway Routing Protocol (IGRP), common interior gateway protocols, cannot support VLSM because they do not send subnet information in their updates. Upon receiving an update packet, these classful routing protocols will use one of the following methods to determine the network prefix of an address:

          If the router receives information about a network, and if the receiving interface belongs to that same network, but on a different subnet, the router applies the subnet mask that is configured on the receiving interface.
          If the router receives information about a network address that is not the same as the one configured on the receiving interface, it applies the default, by class, subnet mask.
          Despite its limitations, RIP is a very popular routing protocol and is supported by virtually all IP routers. The popularity of RIP stems from its simplicity and universal compatibility. However, the first version of RIP, RIP v1, suffers from several critical deficiencies:

          RIP v1 does not send subnet mask information in its updates. Without subnet information, VLSM and CIDR cannot be supported.
          RIP v1 broadcasts its updates, increasing network traffic.
          RIP v1 does not support authentication.
          In 1988, RFC 1058 prescribed the new and improved RIP version 2 (RIP v2) to address these deficiencies:

          RIP v2 does send subnet information and therefore supports VLSM and CIDR.
          RIP v2 multicasts routing updates using the Class D address, providing better efficiency.
          RIP v2 provides for authentication in its updates.
          Because of these key features, RIP v2 should always be preferred over RIP v1 unless some legacy device on the network does not support it.

          When RIP is first enabled on a Cisco router, the router listens for version 1 and 2 updates, but sends only version 1. To take advantage of the RIP v2 features, turn off version 1 support and enable version 2 updates with the following command:

          Router(config)#router rip
          Router(router-config)#version 2

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          • #6
            Re: Cidr & Vlsm

            hello, i am ramji


            • #7
              Re: Cidr & Vlsm

              Hello, I am a Moderator. I am going to ban you for 2 weeks for posting this idiotic message in not only the Cisco Forum but in someones thread!!

              Could someone tell me, is there a BIG flashing sign that is invisible to 99.99% of members that say "Post your Hi/Hello comments anywhere EXCEPT where it tells you in the rules"? (You know, the Coffee Lounge , Hi, Hello, Welcome, Introduction - Newcomers Post HERE thread.)

              AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHHHHHHHH HHHHHHHH I need Milo, and lots of it!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

              Forum RULES. Please read.
              Last edited by biggles77; 6th October 2006, 07:45.
              Joined: 23rd December 2003
              Departed: 23rd December 2015