Why Subnetting Matters
Subnetting is the practice of dividing a network into smaller, more manageable sub-networks. It's essential for efficient IP address management, network security, and reducing broadcast traffic. On the CCNA exam, you'll encounter multiple subnetting questions — and they're worth significant points.
The Basics: IP Addresses and Subnet Masks
An IPv4 address is 32 bits long, written as four octets (e.g., 192.168.1.100). A subnet mask tells you which bits represent the network portion and which represent the host portion. For example, 255.255.255.0 (/24) means the first 24 bits are the network and the last 8 bits are for hosts.
The Magic Number Method
This is the fastest way to subnet. Take the subnet mask octet that's not 0 or 255. Subtract it from 256. That's your "magic number" — the size of each subnet.
Example: Subnet mask 255.255.255.192 (/26). The interesting octet is 192. Magic number = 256 - 192 = 64. So your subnets are: .0, .64, .128, .192 — each with 64 addresses (62 usable hosts).
CIDR Notation Quick Reference
/24 = 256 addresses (254 hosts), /25 = 128 addresses (126 hosts), /26 = 64 addresses (62 hosts), /27 = 32 addresses (30 hosts), /28 = 16 addresses (14 hosts), /29 = 8 addresses (6 hosts), /30 = 4 addresses (2 hosts).
Practice Problems
The only way to master subnetting is through repetition. Start with simple /24 networks and work down to /30. Time yourself — on the CCNA exam, you need to subnet quickly to leave time for other questions. Aim to solve a subnetting problem in under 30 seconds.
Use CCNA Hero's Subnetting module for interactive practice with immediate feedback and detailed explanations for every question.
Common Mistakes to Avoid
Don't forget to subtract the network and broadcast addresses when calculating usable hosts. Remember that the first address in a subnet is the network address and the last is the broadcast address. A /30 subnet has 4 total addresses but only 2 usable — perfect for point-to-point links.