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Cisco CCDP Cisco Exams

642-811 - Building Cisco Multilayer Switched Networks (BCMSN)


642-813 - Implementing Cisco IP Switched Networks


642-873 - Designing Cisco network Service Architectures (ARCH)


642-874 - Designing Cisco Network Service Architectures (ARCH) v2.1


642-902 - Implementing Cisco IP Routing (ROUTE)


Defining Classless Interdomain Routing (CIDR)


Understanding Classless Interdomain Routing (CIDR)

Classless Interdomain Routing (CIDR) or prefix routing is made possible by routing protocols such as OSPF, EIGRP, and RIPv2 that advertise subnet mask information in their routing updates. Prefix routing is the means by which the Internet identifies the portion of the TCP/IP address which in turn identifies the organization.


CIDR addresses the following issues:

  • IP address exhaustion.

  • Exhaustion of network resources needed to manage huge routing tables


RFCs that describe CIDR include:

  • RFC 1517; specifies the applicability statement for implementing CIDR.

  • RFC 1518; specifies architecture for IP address assignment with CIDR.

  • RFC 1519; defines CIDR address assignment and aggregation strategy.

  • RFC 1520; defines routing information over provider boundaries.



With CIDR, aggregates of Class C addresses are assigned by the InterNIC to the different address assignment authorities such as Network Solutions in the United States and R\x8f seaux IP Europ\x8f ens (RIPE) in Europe. The address assignment authorities then divide portions of the addresses between the regional Internet Service Providers (ISPs).


When an organization applies for an IP address, the organization is assigned a group of Class C addresses called a CIDR block.


The CIDR address allocations based on geographic region are shown below:

  • Multiregional: 192.0.0.0 – 193.255.255.255

  • Europe: 194.0.0.0 – 195.255.255.255

  • Others: 196.0.0.0 – 197.255.255.255

  • North America: 198.0.0.0 – 199.255.255.255

  • Central/South America: 200.0.0.0 – 201.255.255.255

  • Pacific Rim: 202.0.0.0 – 203.255.255.255

  • Others: 204.0.0.0 – 205.255.255.255

  • Others: 206.0.0.0 – 207.255.255.255



CIDR starts at /8 and ends at /30. A shorter prefix means that the network is more generally defined, while a longer prefix means that the network is more specifically defined. With the industry standard, you have to add all the bits used in the subnet mask, and show this number as a CIDR. However, the newer IOS running on Cisco routers runs a Setup script which does not prompt you to provide the number of bits used only for subnetting



The benefits of CIDR are summarized here:

  • Reduction in the size of Internet routing tables.

  • More flexibility in terms of assigning IP addresses.

  • Less CPU overhead

  • Less memory overhead

  • Less network traffic



Table 1 shows all the possible CIDR values.


TABLE 1: The CIDR values

Address

CIDR value

255.0.0.0

/8

255.128.0.0

/9

255.192.0.0

/10

255.224.0.0

/11

255.240.0.0

/12

255.248.0.0

/13

255.252.0.0

/14

255.254.0.0

/15

255.255.0.0

/16

255.255.128.0

/17

255.255.192.0

/18

255.255.224.0

/19

255.255.240.0

/20

255.255.248.0

/21

255.255.252.0

/22

255.255.254.0

/23

255.255.255.0

/24

255.255.255.128

/25

255.255.255.192

/26

255.255.255.224

/27

255.255.255.240

/28

255.255.255.248

/29

255.255.255.252

/30






































Table 2 shows how prefix masks are used:


TABLE 2: The usage of prefix masks

Prefix

Mask

New address space

/27

255.255.255.224

12 percent of Class C

30 hosts

/26

255.255.255.192

24 percent of Class C

62 hosts

/25

255.255.255.128

50 percent of Class C

126 hosts

/23

255.255.254.0

2 Class Cs

510 hosts

/22

255.255.252.0

4 Class Cs

1022 hosts

/21

255.255.248.0

8 Class Cs

2046 hosts

/20

255.255.240.0

16 Class Cs

4094 hosts



When configuring a Cisco router with an IP address, you cannot set the number of number of bits utilized in the subnet mask in a router. A router also by default shows CIDR output for the number of bits utilized in the mask.


If you want the router to show the full mask, then use the following command:

  • ip netmask-format command





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