Classless Subnetting and CIDR

Background

• Class-based subnetting has been replaced by classless subnetting since 1993.
• Classless subnetting is also known as Classless Inter-Domain Routing (CIDR).

CIDR Notation

• Allows subnet masks to be assigned that don't fit class A, B, or C.
• Instead of decimal form, subnet masks are denoted by the number of bits, known as CIDR block notation.
  • E.g., Decimal 255.0.0.0 = /8 CIDR.
  • IP address example: 192.168.1.44/24.
  • /24 represents the number of bits in a subnet mask equivalent to 255.255.255.0.

Configuration

• Devices require IP address, subnet mask, default gateway, DNS servers.
• OS often uses decimal masks, while routers/switches may use CIDR notation.
  • E.g., instead of 255.0.0.0, input /8.
• Check device documentation for preferred notation.

Subnet Mask Composition

• Consists of contiguous series of ones (network part) followed by zeros (host part).
• Conversion of binary subnet masks to decimal and CIDR:
  • Example: 255.255.255.0 in binary has 24 ones, equivalent to /24 CIDR.
  • Network address: 24 bits, host address: 8 bits.

Binary to CIDR Conversion

• Example: First two octets all ones, last two all zeros = /16 CIDR.
  • Network: 16 bits, Host: 16 bits.
• Example: Three octets all ones, last octet has two ones and six zeros = /26 CIDR.
  • Network: 26 bits, Host: 6 bits.
• Example: First octet all ones, second octet four ones, rest zeros = /12 CIDR.
  • Network: 12 bits, Host: 20 bits.

Decimal Representation

• Binary subnet masks can be converted to decimal:
  • All zeros = 0.
  • 1 followed by zeros = 128.
  • Two 1s = 192, three 1s = 224, etc.
  • Create a chart for conversion.

Example Conversions

• Subnet mask: All ones in first octet, four ones in second = 255.240.0.0 in decimal, /12 in CIDR.
• New subnet mask example: All ones first octet, three ones second octet = 255.255.224.0, /19 in CIDR.

Reverse Conversion

• Given /26 CIDR, binary = all ones in first three octets, two ones in last.
  • Decimal: 255.255.255.192. Network: 26 bits, Host: 6 bits.
• Given /20 CIDR, first two octets all ones, third four ones = 255.255.240.0.
  • Network: 20 bits, Host: 12 bits.