cce1030 computer networking · classless inter-domain routing (cidr) ip network is represented by a...

CCE1030 Computer

Networking

Lecture 19

Subnetting CIDR / VLSM

Usama Arusi

January 2018 CCE1030 Usama Arusi 1

Lecture Content

Introduction

Classful IP Addressing

Classful Addressing Structure

Classless IP Addressing

CIDR

Classless Routing Protocols

VLSM


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 usable

classes (A, B and C), known as classful IP

addressing

Class A, B, & C addresses were designed to provide

IP addresses for different sized organizations


Classful IP Addressing

Classes of IP addresses are identified by the decimal

number of the 1st octet

10.168.200.34 00001010 10101000 11001000 00100010

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



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



Purpose of a subnet mask

It is used to determine the network portion of an IP address

Classful Routing Updates

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



IP address space was depleting rapidly

the Internet Engineering Task Force (IETF)

introduced Classless Inter-Domain Routing (CIDR)

CIDR (Classless Inter-domain Routing) uses Variable

Length Subnet Masking (VLSM) to help conserve

address space

VLSM is simply subnetting a subnet


Classless IP Addressing

Classless Inter-domain Routing (CIDR) is a way to allow

more flexible allocation of Internet Protocol (IP)

addresses than was possible with the original system

of IP address classes.

Advantage of CIDR:

More efficient use of IPv4 address space. Reduced the problem

of wasted address space

Route summarization, making a flexible way to specify network

addresses in routers


Classless Inter-domain Routing

(CIDR)

IP network is represented by a routing prefix known as CIDR notation

address is written with a suffix indicating the number of bits of the prefix, such as

192.168.2.0/24 – this is an indication of the length of the mask.

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



(CIDR)

Route summarization: A hierarchical architecture where each domain takes its IP addresses

from a higher level.

For example 1: ISP owns network 172.16.0.0/16, then ISP can offer

172.16.1.0/24, 172.16.2.0/24 …. 172.16.254.0/24 to its customers by

sub-netting

ISP only needs to advertise 172.16.0.0/16

Example 2: ISP with customer networks 195.22.0.0/24 to

195.22.15.0/24

Advertise 195.22.0.0/20

Therefore reducing the size of routing table entry



(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



(CIDR)

Steps to calculate a route

summary

1. List networks in binary

format

2. Count number of left most

matching bits to determine

summary route’s mask

3. Copy the matching bits

and add zero bits to

determine the summarized

network address


Classless Routing Protocol

Characteristics of classless routing protocols:

Routing updates include the subnet mask

Supports VLSM

Supports Route Summarization


Classless Routing Protocol

Feature RIP-1 OSPF RIP-2

Classful Yes No No

Classless No Yes Yes

Supports VLSM No Yes Yes

Sends Subnet

MaskNo Yes Yes


Variable Length Subnet Masks

(VLSM)

VLSM & classless routing

Interior routing within an enterprise. This is the process of subnetting a

subnet

Allowing for use of different masks for each subnet

More efficient use of IP addresses as compared to classful IP

addressing


Variable Length Subnet Masks

(VLSM)

VLSM – the process of sub-netting a subnet to fit your needs

Example:

Network 205.15.5.0/24 require the following subnets with supporting hosts.


Subnet # Hosts

A 14

B 28

C 2

D 7

E 28

Subnet # Hosts Network ID Address range Broadcast

B

E

A

D

C

28

28

14

7

2

205.15.5.0/27

205.15.5.32/27

205.15.5.64/28

205.15.5.80/28

205.15.5.96/30

1 - 30

33 - 62

65 - 78

81 - 94

97 - 98

205.15.5.31

205.15.5.63

205.15.5.79

205.15.5.95

205.15.5.99

cce1030 computer networking · classless inter-domain routing (cidr) ip network is represented by a...

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