What Is a Subnet?
A subnet (short for subnetwork) is a logically partitioned section of a larger network. Subnetting divides a block of IP addresses into smaller groups, allowing network administrators to organize devices, limit broadcast traffic, and apply security policies at a granular level. Subnets are expressed in CIDR notation - Use our subnet calculator to break down any CIDR range.
Common Subnet Sizes (IPv4)
| CIDR | Subnet Mask | Total IPs | Usable Hosts | Typical Use |
|---|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 | Large ISP / national network |
| /16 | 255.255.0.0 | 65,536 | 65,534 | Enterprise campus |
| /24 | 255.255.255.0 | 256 | 254 | Typical home or office LAN |
| /25 | 255.255.255.128 | 128 | 126 | Half of a /24, split departments |
| /28 | 255.255.255.240 | 16 | 14 | Small server segment or VLAN |
| /30 | 255.255.255.252 | 4 | 2 | Point-to-point router links |
| /32 | 255.255.255.255 | 1 | 1 (host route) | Single host route, loopback |
The Two Reserved Addresses in Every Subnet
In every subnet, two addresses are reserved and cannot be assigned to hosts: the network address (the first IP, identifying the subnet itself) and the broadcast address (the last IP, used to address all hosts in the subnet simultaneously). For example, in the subnet 192.168.1.0/24, the address 192.168.1.0 is the network address and 192.168.1.255 is the broadcast address, leaving 254 usable host addresses (192.168.1.1 through 192.168.1.254).
Why Subnetting Matters
- Reduces broadcast traffic - Broadcasts only reach devices within the same subnet, not the entire network.
- Improves security - Devices in separate subnets cannot communicate without passing through a router or firewall, enabling access control between departments.
- Enables efficient IP allocation - You assign only as many addresses as needed for each segment.
- Supports VLANs (Virtual LANs) - Each VLAN typically maps to one subnet, creating logical network separation over shared physical infrastructure.
- Required knowledge for configuring port forwarding, firewall rules, and VPN split-tunneling.