© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
ITE PC v4.0
Chapter 1 1
VLSM and CIDR
Routing Protocols and
Concepts – Chapter 6
ITE PC v4.0
Chapter 1 2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 3© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 4© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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)
ITE PC v4.0
Chapter 1 5© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Classful and Classless IP Addressing
 The High Order Bits
These are the leftmost bits in a 32 bit address
ITE PC v4.0
Chapter 1 6© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
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
Range of class C addresses = 192.0.0.0 to 223.255.255.255.
ITE PC v4.0
Chapter 1 7© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 8© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Classful and Classless IP Addressing
ITE PC v4.0
Chapter 1 9© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Classful and Classless IP Addressing
 Purpose of a subnet mask
It is used to determine the network portion of an IP
address
ITE PC v4.0
Chapter 1 10© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 11© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 12© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 13© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Classful and Classless IP Addressing
 Classless Routing Protocol
 Characteristics of classless routing protocols:
-Routing updates include the subnet mask
-Supports VLSM
Supports Route Summarization
ITE PC v4.0
Chapter 1 14© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 15© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 16© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 17© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 18© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 19© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 20© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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)
ITE PC v4.0
Chapter 1 21© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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
ITE PC v4.0
Chapter 1 22© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Summary
 Classless Routing Updates
Subnet masks are included in updates
ITE PC v4.0
Chapter 1 23© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public