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VLSM and CIDR

Routing Protocols and Concepts – Chapter 7

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Objectives

  • Compare and contrast classful and classless IP addressing.
  • Review VLSM and explain the benefits of classless IP addressing.
  • Describe the role of the Classless Inter-Domain Routing (CIDR) standard in making efficient use of scarce IPv4 addresses.

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Introduction

  • Prior to 1981, IP addresses used only the first 8 bits to specify the network portion of the address
  • In 1981, RFC 791 modified the IPv4 32-bit address to allow for three different classes
  • IP address space was depleting rapidly
    • The Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing (CIDR)
      • CIDR uses Variable Length Subnet Masking (VLSM) to help conserve address space
      • VLSM is simply subnetting a subnet

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Classful and Classless IP Addressing

  • Classful IP addressing
  • As of January 2007, there are over 433 million hosts on internet
  • Initiatives to conserve IPv4 address space include:
    • VLSM & CIDR notation (1993, RFC 1519)
    • Network Address Translation (1994, RFC 1631)
    • Private Addressing (1996, RFC 1918)

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Classful and Classless IP Addressing

  • The High Order Bits
    • These are the leftmost bits in a 32 bit address

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Classful and Classless IP Addressing

  • Classes of IP addresses are identified by the decimal number of the 1st octet
    • Class A address begin with a 0 bit
      • Range of class A addresses = 0.0.0.0 to 127.255.255.255
    • Class B address begin with a 1 bit and a 0 bit (10)
      • Range of class B addresses = 128.0.0.0 to 191.255.255.255
    • Class C addresses begin with two 1 bits & a 0 bit (110)
      • Range of class C addresses = 192.0.0.0 to 223.255.255.255

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Classful and Classless IP Addressing

  • The IPv4 Classful Addressing Structure (RFC 790)
    • An IP address has 2 parts:
      • The network portion
        • Found on the left side of an IP address
      • The host portion
        • Found on the right side of an IP address

0 Net Prefix Host ID

class A

class B

class C

class D

class E

10 Net Prefix Host ID

110 Net Prefix Host ID

1110 Multicast address

1111 Research

0 8 16 24 32

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Classful and Classless IP Addressing

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Classful and Classless IP Addressing

  • Purpose of a subnet mask
    • It is used to determine the network portion of an IP address

1111 1111 1111 1111 1111 1111 0000 0000

network mask= 255.255.255.0

/24

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Classful and Classless IP Addressing

  • Classful Routing Updates
    • Recall that classful routing protocols (i.e. RIPv1) do not send subnet masks in their routing updates
    • The reason is that the Subnet mask is directly related to the network address

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Classful and Classless IP Addressing

  • Classless Inter-domain Routing (CIDR – RFC 1517)
    • Advantage of CIDR :
      • More efficient use of IPv4 address space
      • Route summarization
    • Requires subnet mask to be included in routing update because address class is meaningless
    • Recall purpose of a subnet mask:
      • To determine the network and host portion of an IP address

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Classful and Classless IP Addressing

  • Classless IP Addressing
  • CIDR & Route Summarization
    • Variable Length Subnet Masking (VLSM)
    • Allows a subnet to be further sub-netted according to individual needs
    • Prefix Aggregation a.k.a. Route Summarization
    • CIDR allows for routes to be summarized as a single route

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Classful and Classless IP Addressing

  • Classless Routing Protocol
  • Characteristics of classless routing protocols:
    • Routing updates include the subnet mask
    • Supports VLSM
    • Supports Route Summarization

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Classful and Classless IP Addressing

Routing

Protocol

Routing updates

Include subnet

Mask

Supports

VLSM

Ability to send

Supernet routes

Classful

No

No

No

Classless

Yes

Yes

Yes

  • Classless Routing Protocol

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VLSM

  • Classful routing
    • Only allows for one subnet mask for all networks
  • VLSM & Classless routing
    • This is the process of subnetting a subnet
    • More than one subnet mask can be used
    • More efficient use of IP addresses as compared to classful IP addressing

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VLSM

  • VLSM – the process of sub-netting a subnet to fit your needs
  • Example:
    • Subnet 10.1.0.0/16, 8 more bits are borrowed again, to create 256 subnets with a /24 mask.
    • Mask allows for 254 host addresses per subnet
    • Subnets range from: 10.1.0.0 / 24 to 10.1.255.0 / 24

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Plan / Implement a VLSM Addressing Scheme

  • Apply masks from largest group to smallest
  • Avoid assigning addresses that are already allocated
  • Allow for some growth in numbers of hosts on each subnet

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Classless Inter-Domain Routing (CIDR)

  • Route summarization done by CIDR
    • Routes are summarized with masks that are less than that of the default classful mask
    • Example:
      • 172.16.0.0 / 13 is the summarized route for the 172.16.0.0 / 16 to 172.23.0.0 / 16 classful networks

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Classless Inter-Domain Routing (CIDR)

  • Steps to calculate a route summary
    • List networks in binary format
    • Count number of left most matching bits to determine summary route’s mask
    • Copy the matching bits and add zero bits to determine the summarized network address

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Plan a Network Using Classless Routing and CIDR

Route summarization:

  • Use single address to represent group of contiguous subnets
  • Occurs at network boundary
  • Smaller routing table, faster lookups

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Plan a Network Using Classless Routing and CIDR

  • Discontiguous subnets cause unreliable routing
  • Avoid separating subnets with a different network

Route summarize :

192.168.3.0/24

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Plan a Network Using Classless Routing and CIDR

  • Use routing protocols that support VLSM
  • Plan subnetting to complement hierarchical design
  • Disable auto-summarization if necessary
  • Update router IOS
  • Allow for future growth

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Summary

  • Classful IP addressing
    • IPv4 addresses have 2 parts:
      • Network portion found on left side of an IP address
      • Host portion found on right side of an IP address
    • Class A, B, & C addresses were designed to provide IP addresses for different sized organizations
    • The class of an IP address is determined by the decimal value found in the 1st octet
    • IP addresses are running out so the use of Classless Inter Domain Routing (CIDR) and Variable Length Subnet Mask (VLSM) are used to try and conserve address space

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Summary

  • Classful Routing Updates
    • Subnet masks are not sent in routing updates
  • Classless IP addressing
    • Benefit of classless IP addressing
      • Can create additional network addresses using a subnet mask that fits your needs
    • Uses Classless Interdomain Routing (CIDR)

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Summary

  • CIDR
    • Uses IP addresses more efficiently through use of VLSM
      • VLSM is the process of subnetting a subnet
    • Allows for route summarization
      • Route summarization is representing multiple contiguous routes with a single route
  • Classless Routing Updates
    • Subnet masks are included in updates

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