Here’s a concise cheatsheet to help you solve subnetting problems effectively. This summary is designed to cover the key concepts and steps you need, making it a handy reference for tackling such problems.

Subnetting Cheatsheet

1. Understanding IP Addresses and Subnet Masks

• IP Address: A 32-bit address split into network and host portions (e.g., 192.168.1.0).
• Subnet Mask: Defines the network part (1s) and host part (0s). Written as:
  • Dotted decimal (e.g., 255.255.255.0)
  • CIDR notation (e.g., /24, where 24 is the number of network bits).
• Key Idea: The mask determines how many bits are used for the network vs. hosts.

2. Calculating Subnets and Hosts

• Number of Subnets: Borrow s bits from the host portion → Subnets = 2ˢ.
• Number of Hosts per Subnet: With h host bits left → Total addresses = 2ʰ, Usable hosts = 2ʰ - 2 (subtract network and broadcast addresses).

3. Finding the Subnet Mask

• For Subnets: Choose the smallest s where 2ˢ ≥ required number of subnets.
• For Hosts: Choose the smallest h where 2ʰ - 2 ≥ required hosts per subnet.
• Combined: Total bits = 32. If original mask is /n, new mask = /n + s, and host bits h = 32 - new mask.

4. Network and Broadcast Addresses

• Network Address: First address in the subnet (all host bits = 0).
• Broadcast Address: Last address in the subnet (all host bits = 1).
• How to Calculate:
  • Network = IP AND Subnet Mask.
  • Broadcast = Network + (2ʰ - 1).

5. Common Subnet Masks