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2007 Cisco Systems, Inc. All rights reserved. Cisco PublicITE PC v4.0Chapter 1 1

CIDR and VLSM

Erkki Kukk


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ITE PC v4.0Chapter 1 2 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Classful IP Addressing

original specification of IPv4 (RFC 791), released in 1981


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Subnet Mask

Why does a host need to know what network it belongs to? So, it knows whether to encapsulate the IP packet into an Ethernet

frame with:

The Destination MAC Address of the default gatewayMust know the default gateways IP address

The Destination MAC Address of the host with the Destination IPaddress of the packet

Host: Im a host on the 192.168.1.0/24 network.


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When the ARPANET was founded in 1969, no one anticipated that theInternet would explode out of the humble beginnings of this researchproject

Over the next decade, the number of hosts on the Internet grewexponentially, from 159,000 in October 1989 to over 72 million by the end

of the millennium Without the introduction ofVLSM and CIDR notation in 1993 (RFC 1519),

Network Address Translation (NAT) in 1994 (RFC 1631), andprivateaddressingin 1996 (RFC 1918), the IPv4 32-bit address space wouldnow be exhausted


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Classful and Classless Routing Protocols

Routing protocols:classful or classless.

As networks began to use classless addressing, classless routingprotocols had to be modified or developed to include the subnet maskin the routing update.


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Classful IP Addressing

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 thenetwork address


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Moving Toward Classless Addressing

By 1992, members of the IETF had serious concerns about theexponential growth of the Internet and the limited scalability of Internetrouting tables.

They were also concerned with the eventual exhaustion of 32-bit IPv4address space.

1993, IETF introduced classless interdomain routing (CIDR) (RFC 1517).CIDR allowed the following:

More efficient use of IPv4 address space

Prefix aggregation, which reduced the size of routing tables


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CIDR

Classless interdomain routing (CIDR) is a prefix-basedstandard for the interpretation of IP addresses.

CIDR allows routing protocols to summarize multiplenetworks, a block of addresses, as a single route.

With CIDR, IP addresses and their subnet masks arewritten as four octets, separated by periods, andfollowed by a forward slash and a number thatrepresents the subnet mask (slash notation).

An example is 172.16.1.0/24.


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11111111.00000000.00000000.00000000 /8 (255.0.0.0) 16,777,216 host addresses

11111111.10000000.00000000.00000000 /9 (255.128.0.0) 8,388,608 host addresses

11111111.11000000.00000000.00000000 /10 (255.192.0.0) 4,194,304 host addresses

11111111.11100000.00000000.00000000 /11 (255.224.0.0) 2,097,152 host addresses

11111111.11110000.00000000.00000000 /12 (255.240.0.0) 1,048,576 host addresses

11111111.11111000.00000000.00000000 /13 (255.248.0.0) 524,288 host addresses

11111111.11111100.00000000.00000000 /14 (255.252.0.0) 262,144 host addresses

11111111.11111110.00000000.00000000 /15 (255.254.0.0) 131,072 host addresses

11111111.11111111.00000000.00000000 /16 (255.255.0.0) 65,536 host addresses

11111111.11111111.10000000.00000000 /17 (255.255.128.0) 32,768 host addresses

11111111.11111111.11000000.00000000 /18 (255.255.192.0) 16,384 host addresses

11111111.11111111.11100000.00000000 /19 (255.255.224.0) 8,192 host addresses

11111111.11111111.11110000.00000000 /20 (255.255.240.0) 4,096 host addresses

11111111.11111111.11111000.00000000 /21 (255.255.248.0) 2,048 host addresses

11111111.11111111.11111100.00000000 /22 (255.255.252.0) 1,024 host addresses

11111111.11111111.11111110.00000000 /23 (255.255.254.0) 512 host addresses

11111111.11111111.11111111.00000000 /24 (255.255.255.0) 256 host addresses

11111111.11111111.11111111.10000000 /25 (255.255.255.128) 128 host addresses

11111111.11111111.11111111.11000000 /26 (255.255.255.192) 64 host addresses

11111111.11111111.11111111.11100000 /27 (255.255.255.224) 32 host addresses

11111111.11111111.11111111.11110000 /28 (255.255.255.240) 16 host addresses

11111111.11111111.11111111.11111000 /29 (255.255.255.248) 8 host addresses

11111111.11111111.11111111.11111100 /30 (255.255.255.252) 4 host addresses

11111111.11111111.11111111.11111110 /31 (255.255.255.254) 2 host addresses

11111111.11111111.11111111.11111111 /32 (255.255.255.255) Host Route

ISPs no longer restricted to

three classes. Can nowallocate a large range of

network addresses basedon customer requirements


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Classless Routing Protocol

With a classless routing protocol:The /14 (255.252.0.0) subnet mask is included in the routing update

With a classful routing protocol:If R2 sends the 172.16.0.0 summary route without the /14 mask, R3 only knows to apply thedefault classful mask of /16.

Classful routing protocols cannot send supernet routes because the receiving router willapply the default classful mask to the network address in the routing update.


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Classful and Classless IP Addressing

Routing

ProtocolRouting

updates

Includesubnet

Mask

Supports

VLSMAbility to send

Supernet routes

Classful No No No

Classless Yes Yes Yes

Classless Routing Protocol


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Subnetting and Subnet Masks

Formalized in 1985, the subnet mask breaks a

single network in to smaller pieces.

By subnetting we borrow bits from the host part Allows network administrators to divide their network into small networks

orsubnets


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Calculating the number of subnets/hosts needed

Network 172.16.1.0/24 Need:

As many subnets as possible, 60 hosts per subnet

172.16.1.0

Network Host

255.255.255.0


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As many subnets as possible, 60 hosts per subnet New Subnet Mask: 255.255.255.192 (/26)

Number of Hosts per subnet: 6 bits, 64-2 hosts, 62 hosts

Number of Subnets: 2 bits or4 subnets

172.16.1. 0 0 0 0 0 0 0 0

Network Host 6 host bits

255.255.255. 1 1 0 0 0 0 0 0255.255.255.192

Number of subnets


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SubnetNo

Subnet BitsBorrowedBinary Value

SubnetBitsDecimalValue

Host BitsPossible BinaryValues

Subnet/HostDecimalRange

Useable?

0. Subnet 00 0 000000-111111 0-63 No

1. Subnet 01 64 000000-111111 64-127 Yes

2. Subnet 10 128 000000-111111 128-191 Yes

3. Subnet 11 192 000000-111111 192-254 No

172.16.1.0 network172.16.1.64 network172.16.1.128 network172.16.1.192 network



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As many subnets as possible, 12 hosts per subnet

172.16.1.0

Network Host

255.255.255.0


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VLSM

VLSM in ActionVLSM and IP Addresses


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VLSM

VLSM the process ofsub-netting a subnet to fit yourneeds

-Example:

Subnet 10.1.0.0/16, 8 morebits are borrowed again, tocreate 256 subnets with a/24mask.

-Mask allows for 254 hostaddresses per subnet

-Subnets range from:10.1.0.0 / 24 to 10.1.255.0 / 24


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VLSM The network 10.0.0.0/8 has been subnetted using the subnet mask

of /16, which gives the potential of 256 subnets:10.0.0.0/16

10.1.0.0/16

10.2.0.0/16

.

.10.255.0.0/16


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Any of these /16 subnets can be subnetted further. For example the 10.1.0.0/16 subnet is subnetted again using the /

24 mask.

VLSM


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The 10.2.0.0/16 subnet is also subnetted again with a /24 mask. The 10.3.0.0/16 subnet is subnetted again with the /28 mask. The 10.4.0.0/16 subnet is subnetted again with the /20 mask.

VLSM


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VLSM 1


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VLSM 1

255.255.255.240 or /28


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VLSM 2


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VLSM 2


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CIDR and Route Summarization

The 192.168.0.0/20, summarized or aggregated route includes allthe networks belonging to customers A, B, C, and D.


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Route Summarization

Steps to calculate a routesummary

-List networks in binaryformat

-Count number of leftmost matching bits todetermine summaryroutes mask

-Copy the matching

bits and add zero bitsto determine thesummarizednetwork address


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Route Summarization (Example 2)

-Routes are summarized with masks that are less than

that of the default classful mask

-172.16.0.0/13is the summarized route for the172.16.0.0/16 to 172.23.0.0 /16 classful networks


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Lab exercises

Subnetting scenario 1


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SubnettingScenario 1

You have been given the network address 192.168.9.0/24 to subnet and

The network has the following addressing requirements: The BRANCH1 LAN 1 will require 10 host IP addresses. The BRANCH1 LAN 2 will require 10 host IP addresses. The BRANCH2 LAN 1 will require 10 host IP addresses. The BRANCH2 LAN 2 will require 10 host IP addresses. The HQ LAN will require 20 host IP addresses. The link from HQ to BRANCH1 will require an IP address for each end of

the link.

The link from HQ to BRANCH2 will require an IP address for each end ofthe link.

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