Understanding IPv4 Subnetting & CIDR
Subnetting is the process of dividing a single large IP network into smaller, manageable sub-networks (subnets). In modern network architecture and the CCNA 200-301 curriculum, Classless Inter-Domain Routing (CIDR) notation (e.g. /24) defines how many bits are dedicated to the network portion versus the host portion.
The total number of usable hosts in any standard subnet is calculated with the formula: 2(32 - prefix) - 2. The - 2 subtracts the Network ID (all host bits 0) and the Broadcast Address (all host bits 1), which cannot be assigned to individual devices.
Common Subnet Cheat Sheet
| Prefix | Subnet Mask | Total IPs | Usable Hosts | Common Use Case |
|---|---|---|---|---|
| /30 | 255.255.255.252 | 4 | 2 | Point-to-point router links |
| /29 | 255.255.255.248 | 8 | 6 | Small DMZ, WAN blocks |
| /28 | 255.255.255.240 | 16 | 14 | Branch office subnets |
| /27 | 255.255.255.224 | 32 | 30 | Department VLANs |
| /26 | 255.255.255.192 | 64 | 62 | Medium VLAN subnets |
| /24 | 255.255.255.0 | 256 | 254 | Standard LAN / Office Network |
| /16 | 255.255.0.0 | 65,536 | 65,534 | Enterprise core network |