Subnet Mask Basics

• A subnet mask determines which part of an IP address refers to the network and which part refers to the host.
• In binary, the subnet mask is a series of 1s (for the network) followed by 0s (for the host).

Example:

• Subnet Mask (Decimal): 255.255.255.0
• Binary Form: 11111111.11111111.11111111.00000000
• CIDR Notation: /24 (since there are 24 ones)

2. Formula for Calculating Subnet Masks

• Subnet Mask Calculation Formula:Number of Subnets=2n\text{Number of Subnets} = 2^nNumber of Subnets=2n
  • n = number of bits borrowed from the host portion.
• Host Calculation Formula:Usable Hosts per Subnet=2h−2\text{Usable Hosts per Subnet} = 2^h – 2Usable Hosts per Subnet=2h−2
  • h = number of host bits (subtract 2 for network and broadcast addresses)