Classless Interdomain Routing (CIDR)

Classless Interdomain Routing (CIDR)
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

History

Pre 1980:
- 8-bit network IDs
1982:
- RFC790 introduced class-full (or "classful") addressing
- Only included classes A-C
1984:
- Subnetting was introduced (RFC917, RFC950) to reduce wasted space
- A large organization receives a range of addresses and can divide them up into multiple subnets
- A netmask is bit-wise ANDed with an IP address to get the actual address

History (cont.)

1992:
- Supernetting (RFC1338) in 1992 introduced the notion of combining/aggregating class C networks
1993:
- Classless Interdomain Routing (CIDR) - combines subnetting and supernetting into a single standard

CIDR Basics

Obsolete:
- Notion of subnet/supernet
- Subnet 0 restriction
New Notation:
- a.b.c.d/n
  where n denotes the number of bits in the network portion of the address (i.e., the number of 1s that start the mask)

Number of CIDR Addresses

/27 is 1/8 of a C which is 32 hosts
/26 is 1/4 C which is 64
/25 is 1/2 C which is 128
/24 is 1 C which is 256
/23 is 2 C which is 512
/22 is 4 C which is 1024
/21 is 8 C which is 2048
/20 is 16 C which is 4096
/19 is 32 C which is 8192
/18 is 64 C which is 16384
/17 is 128 C which is 32768
/16 is 256 C (which is 1 B) which is 65536
/15 is 512 C which is 131072
/14 is 1024 C which is 262144
/13 is 2048 C which is 524288

Combining Networks

Getting Started:
- I want 1000 addresses
- So, I need to combine 4 Class C networks

An Example of Four Class C Networks:


                           192.60.128.0   (11000000.00111100.10000000.00000000)
192.60.129.0   (11000000.00111100.10000001.00000000)
192.60.130.0   (11000000.00111100.10000010.00000000)
192.60.131.0   (11000000.00111100.10000011.00000000)

An Important Property of these Networks:
- They can be combined using /22

Combining Networks (cont.)

The CIDR Address 192.60.128.0/22:


                           192.60.128.0   (11000000.00111100.10000000.00000000)  Address
255.255.252.0  (11111111.11111111.11111100.00000000)  Mask
192.60.128.0   (11000000.00111100.10000000.00000000)  Supernetted Subnet address
192.60.131.255 (11000000.00111100.10000011.11111111)  Broadcast Address

The CIDR Address 192.60.128.1/22:


                           192.60.128.1   (11000000.00111100.10000000.00000001)  Address
255.255.252.0  (11111111.11111111.11111100.00000000)  Mask
192.60.128.0   (11000000.00111100.10000000.00000000)  Supernetted Subnet address
192.60.131.255 (11000000.00111100.10000011.11111111)  Broadcast Address

Partitioning Networks

Example: Creating Two /24 Networks from a /23

192.168.64.0/23      11000000. 10101000.01000000.00000000

192.168.64.0/24      11000000. 10101000.01000000.00000000
192.168.65.0/24      11000000. 10101000.01000001.00000000

The Hierarchical Relationship

Part of the Hierarchy

There's Always More to Learn