ccna exp2 - chapter06 - vlsm and cidr

8/3/2019 Ccna Exp2 - Chapter06 - Vlsm and Cidr

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Hc vin mng Bch Khoa - Website: www.bkacad.com 2

Objectives

Compare and contrast classful and classless IPaddressing.

Review VLSM and explain the benefits of classless IPaddressing.

Describe the role of the Classless Inter-DomainRouting (CIDR) standard in making efficient use ofscarce IPv4 addresses


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Introduction

Prior to 1981, IP addresses used only the first 8 bits tospecify the network portion of the address

In 1981, RFC 791 modified the IPv4 32-bit address toallow 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 oninternet

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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The High Order BitsThese are the leftmost bits in a 32 bit address



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Classes of IP addresses are identified by the decimalnumber 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.



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The IPv4 Classful Addressing Structure (RFC 790) An IP address has 2 parts:

The network portion: Found on the left side of an IPaddress

The host portion: Found on the right side of an IPaddress




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




Classless Inter-domain Routing (CIDR RFC 1517)

Allows for:

More efficient use of IPv4 address space.

Route summarization.

Requires subnet mask to be included in routing update

because address class is meaningless.





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.




Classless Routing Protocol Characteristics of classless routing protocols:

- Routing updates include the subnet mask

- Supports VLSM

- Supports Route Summarization




RoutingProtocol

RoutingupdatesInclude

subnetMask

SupportsVLSM

Ability to sendSupernet routes

Classful No No No

Classless Yes Yes Yes

Classless Routing Protocol



VLSM

Classful routing Only allows for one

subnet mask for allnetworks

VLSM & classless routing This is the process of

subnetting a subnet

More than one subnet

mask can be used.



207.21.24.192/30

207.21.24.196/30 207.21.24.200/30

207.21.24.204/30

207.21.24.208/30 207.21.24.212/30

207.21.24.32/27

207.21.24.64/27

207.21.24.96/27 207.21.24.128/27

207.21.24.160/27 207.21.24.224/27 207.21.24.0/27

207.21.24.216/30

This network has seven /27 subnets with 30 hosts each ANDseven/30 subnets with 2 hosts each (one left over).

/30 subnets with 2 hosts per subnet do not waste host addresses onserial networks .



VLSM and the Routing Table

Routing Table without VLSM

RouterX#show ip route

207.21.24.0/27 is subnetted, 4 subnets

C 207.21.24.0 is directly connected, Serial0



C 207.21.24.96 is directly connected, FastEthernet0

Routing Table with VLSM

RouterX#show ip route

207.21.24.0/24 is variably subnetted, 4 subnets, 2 masks

C 207.21.24.192 /30 is directly connected, Serial0



C 207.21.24.96 /27 is directly connected, FastEthernet0

Parent Route shows classful mask instead of subnet mask of the childroutes.

Each Child Routes includes its subnet mask.

Displays one subnet mask for all child routes.

Classful mask is assumed for the parent route.

Each child routes displays its own subnet mask.Classful mask is included for the parent route.



VLSM

VLSM the process ofsub-netting a subnet tofit your needs

- Example:

Subnet 10.1.0.0/16, 8

more bits are borrowedagain, to create 256subnets with a /24 mask.

- Mask allows for 254

host addresses persubnet

- Subnets range from:10.1.0.0 / 24 to

10.1.255.0 / 24



All Zeros and All Ones Subnets

There is no command to enable or disable the use of the all-ones subnet,

it is enabled by default.

Router(config)#ip subnet-zero

The use of the all-ones subnet has always been explicitly allowed and

the use of subnet zero is explicitly allowed since Cisco IOS version 12.0.

RFC 1878 states, "This practice (of excluding all-zeros and all-ones

subnets) is obsolete! Modern software will be able to utilize all definable

networks." Today, the use of subnet zero and the all-ones subnet is

generally accepted and most vendors support their use, though, on

certain networks, particularly the ones using legacy software, the use of

subnet zero and the all-ones subnet can lead to problems.



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 the172.16.0.0 /16 to 172.23.0.0 /16 classful networks

Classless Inter-Domain Routing (CIDR)



Classless Inter-Domain Routing (CIDR)

Steps to calculate a routesummary List networks in binary

format

Count number of left

most matching bits todetermine summaryroutes mask

Copy the matching bitsand add zero bits to

determine thesummarized networkaddress



Without CIDR, arouter mustmaintainindividualrouting table

entries for theseclass Bnetworks.

With CIDR, a

router cansummarizethese routesinto eightnetworks byusing a 13-bit

prefix:172.24.0.0 /13

1. Count the number of left-most matching bits, /13

2. Add all zeros after the last matching bit:

172.24.0.0 = 10101100 00011000 00000000 00000000

Steps:



CIDR (Classless Inter-Domain Routing)

By using a prefix address to summarizes routes,administrators can keep routing table entries manageable,which means the following

More efficient routing

A reduced number of CPU cycles when recalculating arouting table, or when sorting through the routing tableentries to find a match

Reduced router memory requirements

Route summarization is also known as: Route aggregation

Supernetting

Supernetting is essentially the inverse of subnetting.



Company XYZ needs to address 400 hosts. Its ISP gives them two contiguous Class C addresses:

207.21.54.0/24

207.21.55.0/24

Company XYZ can use a prefix of 207.21.54.0 /23 to supernetthese two contiguous networks. (Yielding 510 hosts)

207.21.54.0 /23 207.21.54.0/24

207.21.55.0/24

23 bits in common

Supernetting Example



With the ISP acting as the addressing authority for a CIDR block ofaddresses, the ISPs customer networks, which include XYZ, can be

advertised among Internet routers as a single supernet.

Supernetting Example


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CIDR and the provider


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200.199.48.0/24 11001000 11000111 001100 00 00000000200.199.49.0/24 11001000 11000111 001100 01 00000000200.199.50.0/24 11001000 11000111 001100 10 00000000200.199.51.0/24 11001000 11000111 001100 11 00000000200.199.48.0/22 11001000 11000111 001100 00 00000000

200.199.52.0/24 11001000 11000111 001101 00 00000000200.199.53.0/24 11001000 11000111 001101 01 00000000200.199.54.0/24 11001000 11000111 001101 10 00000000200.199.55.0/24 11001000 11000111 001101 11 00000000200.199.52.0/22 11001000 11000111 001101 00 00000000

200.199.56.0/24 11001000 11000111 00111 000 00000000200.199.57.0/24 11001000 11000111 00111 001 00000000..

200.199.63.0/24 11001000 11000111 00111 111 00000000200.199.56.0/21 11001000 11000111 00111 000 00000000

CIDR and the provider


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Dynamic routing protocols must send network addressandmask(prefix-length) information in their routing updates.

In other words, CIDR requires classless routing protocols for dynamic

routing.

CIDR Restrictions


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Route flapping

Route flapping occurs when a router interface alternates rapidly between theup and down states.

Route flapping, and it can cripple a router with excessive updates andrecalculations.

However, the summarization configuration prevents the RTC route flappingfrom affecting any other routers.

The loss of one network does not invalidate the route to the supernet. While RTC may be kept busy dealing with its own route flap, RTZ, and all

upstream routers, are unaware of any downstream problem.

Summarization effectively insulates the other routers from the problem of routeflapping.


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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 InterDomain Routing (CIDR) and Variable Length Subnet Mask (VLSM)are used to try and conserve address space.


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Classful Routing Updates Subnet masks are not sent in routing updates Classless IP addressing

Subnet masks are included in updates

Benefit of classless IP addressing Can create additional network addresses using a

subnet mask that fits your needs.

Saving IP address space.

Uses Classless Interdomain Routing (CIDR).

Summary


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CIDR

Uses IP addresses more efficiently throughuse of VLSM

VLSM is the process of subnetting a subnet.

Allows for route summarization

Route summarization is representingmultiple contiguous routes with a single

route.

Summary


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