Subnet Calculator

You type an IPv4 address and choose a CIDR prefix, like /24, and it works out the rest. It builds the subnet mask from the prefix, lines up the address against it with bitwise math, and reads off the network address, the broadcast address, the first and last usable host, the host count, and the wildcard mask. It all runs on any device, so you'll see every value update the moment you change the address or the prefix.

CIDR, short for Classless Inter-Domain Routing, writes a subnet as the address followed by a slash and a number, like 192.168.1.0/24. That number is the prefix length, the count of leading bits fixed for the network, so /24 means the first 24 bits are the network and the last 8 are for hosts. A bigger prefix means a smaller subnet with fewer hosts, which is why /30 leaves room for just a couple of devices.

Take 2 to the power of the host bits, which is 32 minus the prefix, then subtract 2 for the network and broadcast addresses that you cannot assign. A /24 has 8 host bits, so 2 to the 8 is 256, minus 2 leaves 254 usable hosts. A /26 has 6 host bits, giving 64 total and 62 usable. The calculator does that for you and shows both the total and the usable count.

The network address is the first address in the subnet, with all host bits set to zero, and it names the subnet itself. The broadcast address is the last address, with all host bits set to one, and a packet sent there reaches every host on the subnet. Neither is assigned to a device, which is why the usable range sits between them, from the network plus one to the broadcast minus one.

A wildcard mask is the inverse of the subnet mask, with the bits flipped, so a mask of 255.255.255.0 becomes a wildcard of 0.0.0.255. Where a 0 in the wildcard means the bit must match, a 1 means anything goes in that spot. Access control lists on routers, especially Cisco gear, use wildcard masks rather than subnet masks, so it's handy to see both side by side here, and you won't have to flip the bits in your head.

You borrow bits from the host part. Pick how many equal subnets you want, and the tool works out the new prefix as the old prefix plus the log base 2 of that count, so a /24 split into 4 becomes four /26 blocks. Each block gets 2 to the power of the remaining host bits addresses, and the table lists every subnet with its own network, usable range, and broadcast. It's the fast way to carve a /24 into VLANs without stepping through the binary by hand.

Yes, there's a basic IPv6 panel. Enter an address like 2001:db8::1 and a prefix such as /64, and it returns the network prefix, the first and last address in the block, and the total address count. IPv6 has no broadcast address and no need to reserve the first and last host, so a /64 holds 2 to the 64 addresses, all usable. The count is computed with big-integer math so even a /48 or /32 shows an exact figure.

Yes, it's completely free, with no sign-up, and it runs right in your browser, so no addresses you enter leave your device. Type an IP, pick a prefix, and read the full breakdown in a tap. Bookmark it for network planning, splitting a block into VLANs, studying for a certification, or a homework problem, and you'll have an accurate subnet breakdown whenever you need it.