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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 1^st 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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