Subnet Calculator

Subnetting is the practice of dividing a single physical network into multiple smaller logical networks called subnets. It improves security by isolating broadcast domains, reduces network congestion, and makes IP address management far more efficient. At the heart of subnetting is the bitwise AND operation: a device takes its own IP address and performs a bitwise AND with the subnet mask to determine its local network prefix.

Before CIDR, the internet used rigid class-based addressing. Class A networks (/8) supported 16 million hosts, Class B (/16) supported 65,000, and Class C (/24) supported 254. This wasted enormous blocks of addresses. CIDR and Variable Length Subnet Masking (VLSM) solved this by allowing arbitrary prefix lengths, letting administrators carve out exactly the right amount of address space for each network segment. For a quick reference of all CIDR values, see our Subnet Mask Table.

In IPv6, subnetting is dramatically simpler. The standard LAN allocation is a /64 subnet, providing 2⁶⁴ addresses—more than enough for every device on Earth. Because the address space is so vast, IPv6 networks typically do not use NAT, and each device can have a globally unique routable address while still being protected by stateful firewalls.