Definition
A subnet is a logical subdivision of an IP network. Devices in the same subnet can communicate directly without going through a router; communication between subnets requires routing. A subnet is defined by a network address (the start of the range) and a subnet mask (which bits are the network portion versus the host portion). For example, 192.168.1.0/24 is a subnet containing 256 addresses from 192.168.1.0 to 192.168.1.255.
Subnets in VPNs
VPN tunnels carve out their own private IP ranges. When you connect, your device receives an internal VPN IP from a subnet the provider operates. Routing rules decide which traffic uses the VPN tunnel: typically all of it, except for local network subnets (e.g., 192.168.0.0/16, 10.0.0.0/8) so you can still reach printers and other local devices. Site-to-site VPNs explicitly define which subnets on each side are reachable through the tunnel.
Why It Matters to Users
Most users never think about subnets directly, but subnetting decisions affect VPN behaviour. If your home network's subnet (e.g., 192.168.1.0/24) overlaps with the corporate network's subnet, you cannot reach corporate resources through the VPN — the routing is ambiguous. This is why corporate IT often standardizes on uncommon private subnets (172.20.x.x, 10.50.x.x) that are unlikely to overlap with anyone's home network.