CIDR (Classless Interdomain Routing)

Classless Interdomain Routing (CIDR) was introduced to improve both address space utilization and routing scalability in the Internet.

It was needed because of the rapid growth of the Internet and growth of the IP routing tables held in the Internet routers.

CIDR moves way from the traditional IP classes (Class A, Class B, Class C, and so on). In CIDR , an IP network is represented by a prefix, which is an IP address and some indication of the length of the mask.

Length means the number of left-most contiguous mask bits that are set to one. So network 172.16.0.0/255.255.0.0. can be represented as 172.16.0.0/16. CIDR also depicts a more hierarchical Internet architecture, where each domain takes its IP addresses from a higher level. This allows for the summarization of the domains to be done at the higher leveL.

For example, if an ISP owns network 172.16.0.0/16, then the ISP can offer 172.16.1.0/24, 172.16.2.0/24, and so on to customers. Yet, when advertising to other providers, the ISP only needs to advertise 172.16.0.0/16.

Sample Config

Routers A and B are connected via serial interface.

Router A

   hostname router'a'

  !

  ip routing

  !int e 0

  ip address 172.16.50.1 255.255.255.0

  !(subnet 50)

  int e 1 ip address 172.16.55.1 255.255.255.0

  !(subnet 55)

  int t 0 ip address 172.16.60.1 255.255.255.0

  !(subnet 60) int s 0

  ip address 172.16.65.1 255.255.255.0 (subnet 65)

  !s 0 connects to router B

  router rip

  network 172.16.0.0

Router B

  hostname router'b'

  !

  ip routing

  !int e 0

  ip address 192.1.10.200 255.255.255.240

  !(subnet 192)

  int e 1

  ip address 192.1.10.66 255.255.255.240

  !(subnet 64)

  int s 0

  ip address 172.16.65.2 (same subnet as router A's s 0)

  !Int s 0 connects to router A

  router rip

  network 192.1.10.0

  network 172.16.0.0

Host/Subnet Quantities Table:

Class B                   Effective  Effective

# bits        Mask         Subnets     Hosts

-------  ---------------  ---------  ---------

  1      255.255.128.0           2     32766

  2      255.255.192.0           4     16382

  3      255.255.224.0           8      8190

  4      255.255.240.0          16      4094

  5      255.255.248.0          32      2046

  6      255.255.252.0          64      1022

  7      255.255.254.0         128       510

  8      255.255.255.0         256       254

  9      255.255.255.128       512      126

  10     255.255.255.192      1024     62

  11     255.255.255.224      2048     30

  12     255.255.255.240      4096     14

  13     255.255.255.248      8192     6

  14     255.255.255.252     16384    2

Class C                   Effective  Effective

# bits        Mask         Subnets     Hosts

-------  ---------------  ---------  ---------

  1      255.255.255.128      2        126

  2      255.255.255.192      4         62

  3      255.255.255.224      8         30

  4      255.255.255.240     16         14

  5      255.255.255.248     32          6

  6      255.255.255.252     64          2

 

*Subnet all zeroes and all ones included. These

 might not be supported on some legacy systems.

*Host all zeroes and all ones excluded.