Chapter 6 - VLSM and CIDR CCNA docx

Học viện mạng Bách Khoa - Website: www.bkacad.com 3Introduction • Prior to 1981, IP addresses used only the first 8 bits to specify the network portion of the address • In 1981, RFC 791

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Chapter 6 - VLSM and CIDR

CCNA Exploration version 4.0

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Học viện mạng Bách Khoa - Website: www.bkacad.com 2

• 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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• 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

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• 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 isdirectly related to the network address

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• 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

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• 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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• Classless Routing Protocol

• Characteristics of classless routing protocols:

- Routing updates include the subnet mask

- Supports VLSM

- Supports Route Summarization

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Routing

Protocol

Routing updates Include subnet Mask

Supports VLSM

Ability to send Supernet routes

• 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

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• 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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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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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.

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• Route summarization done by CIDR

– Routes are summarized with masks that are less

than that of the default classful mask

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

2 Add all zeros after the last matching bit:

172.24.0.0 = 10101100 00011 000 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)

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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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• Dynamic routing protocols must send network address and mask

(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 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

• 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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• 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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use of VLSM

multiple contiguous routes with a single route.

Summary

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