Introduction to CIDR Blocks

Acronyms

CIDR - Classless Inter-Domain Routing

What is CIDR?

Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and IP routing. CIDR notation is a compact representation of an IP address and its associated routing prefix. The notation is constructed from an IP address, a slash (’/’) character, and an integer.

CIDR Notation

An IPv4 address has 4 parts (e.g 192.168.100.0), each of which is 8 bits. Hence an IPv4 address is 32 bits in total. On the other hand, a CIDR block may be represented thus: 192.168.100.14/24 The integer 24 after the slash / tells us that 24 out of 32 bits of the ip address identify the network, wile 8 out of 32 bits identify the host.

• 192.168.100.14/24 represents the IPv4 address 192.168.100.14 and its associated routing

prefix 192.168.100.0, or equivalently, its subnet mask 255.255.255.0, which has 24 leading 1-bits.

• Formula for calculating number of assignable IP addresses = 2 ^ (32 - n) - 2.

  • Where n = number of network bits (number after /)
  • 151.0.0.0/28 has 28 network bits and 2 ^ (32 - 28) - 2 = 14 IP addresses.
  • We subtract 2 for the network and broadcast addresses.

• The IPv4 block 192.168.100.0/22 represents 1024 IPv4 addresses from 192.168.100.0 to 192.168.103.255.

  • 22 network bits implies 2 ^ (32 - 22) - 2 = 1022 (remember 2 subtracted for network and broadcast addresses)

  • Addresses are 255 per section of IP address i.e:

    • 192.168.100.[0-255] = 256 total
    • 192.168.101.[0-255] = 512 total cumulative
    • 192.168.102.[0-255] = 768 total cumulative
    • 192.168.103.[0-255] = 1024 total cumulative

• The IPv6 block 2001:db8::/48 represents the block of IPv6 addresses from

2001:db8:0:0:0:0:0:0 to 2001:db8:0:ffff:ffff:ffff:ffff:ffff.

• ::1/128 represents the IPv6 loopback address. Its prefix length is 128 which is the number

of bits in the address.

For IPv4, CIDR notation is an alternative to the older system of representing networks by their starting address and the subnet mask, both written in dot-decimal notation. 192.168.100.0/24 is equivalent to 192.168.100.0/255.255.255.0.

References

• Wikipedia - Class Inter-Domain Routing