What this tool does
Enter any IPv4 address and CIDR prefix to instantly get the network address, broadcast address, subnet mask, and the full usable host range — the same math you'd otherwise do by hand or in a spreadsheet during VLAN planning or firewall rule design.
How subnetting works
An IPv4 address is 32 bits, split into a network portion and a host portion by the CIDR prefix (the /24, /27, etc.). Every address in a subnet shares the same network portion; the host portion is what varies between individual devices. A /24 has 8 host bits (256 total addresses), a /27 has 5 host bits (32 total addresses), and each additional prefix bit halves the subnet size.
When you'd use this
Designing a new VLAN, sizing a DHCP scope, checking whether two subnets overlap before a merger or site migration, or working out how many usable IPs a given prefix actually gives you before requesting address space from your network team.
Reading the results
The network address identifies the subnet itself and can't be assigned to a device. The broadcast address is reserved for sending to every host on the subnet at once. Everything in between — the usable host range — is what you can actually assign to routers, servers, and clients.
CIDR notation vs. dotted-decimal subnet mask
CIDR notation (`/24`) and a dotted-decimal mask (`255.255.255.0`) describe exactly the same thing -- how many bits of an address are the network portion -- just written differently. A `/24` means the first 24 bits are fixed (the network), which as a dotted-decimal mask is `255.255.255.0`. A `/27` (5 host bits) is `255.255.255.224`. This tool accepts CIDR input and shows you the equivalent mask, so you don't need to memorize the conversion table.
VLSM in short
Variable Length Subnet Masking just means not every subnet on a network has to be the same size -- a point-to-point WAN link genuinely only needs a /30 (2 usable addresses), while a user VLAN might need a /23 or larger. Sizing each subnet to what it actually needs, rather than using one fixed size everywhere, is what keeps a larger address plan from running out of room.