Classless Subnetting and CIDR

Transition from Class-based to Classless Subnetting

• Pre-1993: IP addresses had class-based subnetting (Class A, B, C).
• Post-1993: Shift to classless subnetting, known as Classless Inter-Domain Routing (CIDR).
• CIDR allows subnet masks that do not conform to Class A, B, or C.

CIDR Block Notation

• Subnet masks are expressed in terms of bits, not decimals.
• CIDR block notation: e.g., /24 means 24 bits are part of the subnet mask.
• Example: IP 192.168.1.44/24 refers to 24 bits in the subnet mask.

Configuration Practices

• Different devices may require different notations for subnet masks (decimal vs. CIDR block).
  • Operating Systems: Often use decimal notation.
  • Routers/Switches: May use CIDR block notation.
• Always check device documentation for preferred subnet mask format.

Structure of a Subnet Mask

• Subnet masks are a series of contiguous 1s followed by 0s.
  • Example: Three octets of 1s (24 1s) means a /24 CIDR notation.
  • Purpose: Separates the network part from the host part of the address.

Converting Binary to CIDR Block

• Count the number of 1s to determine the CIDR notation.
  • Example: Two octets of 1s (16 1s) = /16.
  • Example: Three octets of 1s and two 1s in fourth octet (26 1s) = /26.
• Slash notation indicates network bits vs. host bits.

Conversion to Decimal

• Binary 1s and 0s convert into decimals for easier understanding.
  • Single 1 = 128, Two 1s = 192, Three 1s = 224, etc.
• Create a chart for binary-to-decimal conversion of subnet masks.
  • Example: /12 corresponds to 255.240.0.0 in decimal.

Examples of CIDR and Decimal Conversion

• Example 1: All 1s in first octet, four 1s in second octet: /12.
  • Decimal: 255.240.0.0
• Example 2: All 1s in first and second octet, three 1s in third octet: /19.
  • Decimal: 255.255.224.0
• Reverse Conversion: Given /26, binary is three full octets of 1s and two 1s in the last octet.
  • Decimal: 255.255.255.192

Practical Applications

• Use CIDR for determining network and host sections of addresses efficiently.
• Convert between CIDR and decimal as needed for configuration and understanding.