Lecture Notes: Classless Subnetting and CIDR Notation

Overview

• Transition from class-based to classless subnetting since 1993.
• Classless subnetting allows for flexible subnet mask allocation beyond traditional Class A, B, C.
• Uses CIDR (Classless Inter-Domain Routing) notation.

Key Concepts

CIDR Notation

• Subnet masks are expressed as a count of leading 1 bits.
• Example:
  • Decimal 255.0.0.0 equivalent to /8 in CIDR.
  • 192.168.1.44/24 implies a subnet mask of 255.255.255.0.

Configuration Practices

• Devices often require subnet mask in different formats:
  • Operating systems (e.g., Windows) may require decimal format.
  • Routers/switches may use CIDR notation.
• Check device documentation for format preferences.

Subnet Masks

Binary and Decimal Conversion

• Subnet masks use contiguous 1s followed by 0s.
• Example Conversion:
  • 11111111.11111111.11111111.00000000 to Decimal: 255.255.255.0 and CIDR: /24

Examples of CIDR Conversion

Example 1:

• Subnet with all 1s in first two octets.
• Binary: 11111111.11111111.00000000.00000000
• Decimal: 255.255.0.0
• CIDR: /16

Example 2:

• Subnet with first three octets all 1s, and two 1s in fourth octet.
• Binary: 11111111.11111111.11111111.11000000
• Decimal: 255.255.255.192
• CIDR: /26

Example 3:

• Subnet with first octet all 1s, second octet with four 1s.
• Binary: 11111111.11110000.00000000.00000000
• Decimal: 255.240.0.0
• CIDR: /12

CIDR Notation to Decimal

• Reference chart for binary to decimal conversion for each octet.
• Example:
  • /19 translates to binary 11111111.11111111.11100000.00000000
  • Decimal: 255.255.224.0

Application and Reverse Calculation

Calculating from CIDR to Binary

• Example: /26 implies three full octets of 1s and two 1s in the fourth octet.
• Binary: 11111111.11111111.11111111.11000000
• Decimal: 255.255.255.192

Reverse Example

• Given CIDR /20, convert to binary and decimal.
• Binary: 11111111.11111111.11110000.00000000
• Decimal: 255.255.240.0

Conclusion

• CIDR notation simplifies subnetting by focusing on the number of 1s in the subnet mask.
• Understanding binary to decimal conversion is crucial for network configuration.
• Use charts for efficient conversion between formats.