Subnet Calculator

The number after the slash is how many leading bits of the address are the network part. A /24 fixes the first 24 bits, leaving 8 for hosts: 256 addresses, 254 of them usable. Smaller numbers mean bigger networks; a /16 holds 65,536 addresses.

The lowest address names the network itself and the highest is the broadcast address, so a /24 offers 254 usable hosts rather than 256. The exceptions are /31, defined by RFC 3021 for point-to-point links with both addresses usable, and /32, a single host route.

Three ranges are reserved for private use by RFC 1918: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. They are not routable on the public internet, which is why they appear in almost every office network and cloud VPC. The calculator labels these plus loopback, link-local, carrier-grade NAT and multicast ranges.

Start with the VPC block, say a /16, then use the split section to divide it at a longer prefix. A common pattern is one pair of subnets per availability zone, public and private, sized to leave room to grow. Cloud platforms also reserve a few addresses per subnet beyond the usual two, so leave headroom.

The bitwise inverse of the netmask, used by router access lists and some firewall rules. Where the netmask for a /24 is 255.255.255.0, the wildcard is 0.0.0.255.

No. All calculation happens in your browser and the page keeps working offline. Address plans are sensitive infrastructure detail, and they stay on your machine.