ca ex s2m06 vlsm and cidr

CCNA – Semester 2

Chapter 6: VLSM and CIDR

CCNA Exploration version 4.0

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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 Addressing

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• 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)

Classful IP Addressing

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• The High Order Bits

These are the leftmost bits in a 32 bit address

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• 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


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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 IP address

• The host portion

– Found on the right side of an IP address


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• 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

Classful Routing Protocol

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Classless IP Addressing

• 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

Recall purpose of a subnet mask:

– To determine the network and host portion of an IP address

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• 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

CIDR and Route Summarization

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Classless Routing Protocol

• Characteristics of classless routing protocols:

– Routing updates include the subnet mask

– Supports VLSM

– Supports Route Summarization

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Classless Routing Protocol

Routing

Protocol

Routing

updates

Include subnet

Mask

Supports

VLSM

Ability to

send

Supernet

routes

Classful No No No

Classless Yes Yes Yes

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VLSM

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• 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

VLSM is Action

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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 AND seven /30 subnets with 2 hosts each (one left over).

• /30 subnets with 2 hosts per subnet do not waste host addresses on serial networks .

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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 child routes.

• 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.

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• 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

VLSM

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All Zeros and All Ones Subnets

Using the All Ones Subnet

• There is no command to enable or disable the use of the all-ones subnet, it is enabled by default.

• 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.

Router(config)#ip subnet-zero

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.

CCO: Subnet Zero and the All-Ones Subnet

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a

0080093f18.shtml

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CIDR

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• 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

Route Summarization

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Calculating Route Summarization

• 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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Without CIDR, a router must maintain individual routing table entries for these class B networks.

With CIDR, a router can summarize these routes into eight networks by using 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:

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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

a routing table, or when sorting through the routing table

entries to find a match

– Reduced router memory requirements

• Route summarization is also known as:

– Route aggregation

– Supernetting

• Supernetting is essentially the inverse of subnetting.

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• 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 supernet

these 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

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• With the ISP acting as the addressing authority for a CIDR block of

addresses, the ISP’s 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 00000000

200.199.49.0/24 11001000 11000111 001100 01 00000000

200.199.50.0/24 11001000 11000111 001100 10 00000000

200.199.51.0/24 11001000 11000111 001100 11 00000000

200.199.48.0/22 11001000 11000111 001100 00 00000000

200.199.52.0/24 11001000 11000111 001101 00 00000000

200.199.53.0/24 11001000 11000111 001101 01 00000000

200.199.54.0/24 11001000 11000111 001101 10 00000000

200.199.55.0/24 11001000 11000111 001101 11 00000000

200.199.52.0/22 11001000 11000111 001101 00 00000000

200.199.56.0/24 11001000 11000111 00111 000 00000000

200.199.57.0/24 11001000 11000111 00111 001 00000000

…………..

200.199.63.0/24 11001000 11000111 00111 111 00000000

200.199.56.0/21 11001000 11000111 00111 000 00000000

CIDR and the provider

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• Dynamic routing protocols must send network address and mask (prefix-length) information in their routing updates.

• In other words, CIDR requires classless routing protocolsfor dynamic routing.

CIDR Restrictions

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Route flapping

• Route flapping occurs when a router interface alternates rapidly between the up and down states.

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

• However, the summarization configuration prevents the RTC route flapping from 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 route flapping.

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Summary

ca ex s2m06 vlsm and cidr

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