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Classless Interdomain Routing (CIDR)
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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History
Back SMYC Forward
  • Pre 1980:
    • 8-bit network IDs
  • 1982:
    • RFC790 introduced class-full (or "classful") addressing
    • Only included classes A-C
      ip-addresses
  • 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.)
Back SMYC Forward
  • 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
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  • 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)
    • cidr-addresses
Number of CIDR Addresses
Back SMYC Forward
  • /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
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  • 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.)
Back SMYC Forward
  • 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
Back SMYC Forward

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
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Part of the Hierarchy

cidr_partial-tree
There's Always More to Learn
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