CIDR (Classless Inter-Domain Routing) was introduced to overcome the limitations of the existing class-based IP allocation. It allows flexible allocation of IP addresses as needed, preventing address waste.

CIDR Notation and Structure

CIDR represents the network bit count with the ‘/’ symbol followed by the IP address:

  • 192.168.1.0/24

    • Network part: 192.168.1 (24 bits)
    • Host part: Last 8 bits
    • Available IPs: 254

  • 192.168.1.0/25

    • Network part: 192.168.1.0 (25 bits)
    • Host part: Last 7 bits
    • Available IPs: 126

Relation with Subnet Mask

CIDR prefixes correspond one-to-one with subnet masks:

  • /24 = 255.255.255.0

    • Binary: 11111111.11111111.11111111.00000000
    • Available IPs: 254

  • /25 = 255.255.255.128

    • Binary: 11111111.11111111.11111111.10000000
    • Available IPs: 126

  • /26 = 255.255.255.192

    • Binary: 11111111.11111111.11111111.11000000
    • Available IPs: 62

Understanding Network Size

The network size based on the CIDR prefix is calculated as a power of 2:

  • /24 = 2^8 = 256 addresses
  • /25 = 2^7 = 128 addresses
  • /26 = 2^6 = 64 addresses
  • /27 = 2^5 = 32 addresses

The first and last addresses in each network are reserved for network and broadcast addresses and are not available for actual use.