Convert a CIDR prefix into its dotted-decimal subnet mask, free and instant.
CIDR notation (like 192.168.1.0/24) packs a network address and its size into one compact string — the number after the slash is the prefix length, meaning how many of the 32 bits in an IPv4 address are fixed as the network portion, with the rest available for host addresses.
This tool expands that notation into everything it implies: the network address, the broadcast address, the subnet mask in standard dotted-decimal form, the first and last usable host addresses, and the total and usable address counts. A smaller prefix number (like /8) covers a huge address range; a larger one (like /30) covers just 4 addresses — the relationship is inverse and easy to get backwards under time pressure.
Two special cases are handled correctly per RFC 3021: a /31 network (used for point-to-point links) treats both addresses as usable rather than reserving one for network and one for broadcast, and a /32 is treated as a single host address.
Worked example: 192.168.1.0/24 has 32 − 24 = 8 host bits remaining, giving 2⁸ = 256 total addresses in the range from 192.168.1.0 to 192.168.1.255. Of those, the first (192.168.1.0) is reserved as the network address and the last (192.168.1.255) as the broadcast address, leaving 254 usable addresses for actual devices — 192.168.1.1 through 192.168.1.254. The subnet mask 255.255.255.0 is just another way of writing the same /24 prefix: 24 bits set to 1, then 8 bits set to 0.
Two addresses in every standard subnet are reserved — the network address itself (all host bits zero) and the broadcast address (all host bits one) — leaving 256 minus 2 for actual hosts. The exceptions are /31 and /32, handled per RFC 3021.
RFC 3021 defines /31 specifically for point-to-point links, where both addresses are usable (no network/broadcast reservation) since there are only ever two devices on the link.
No — this calculation is pure arithmetic on the numbers you enter, done entirely in your browser.
Both — the output includes the network address, broadcast address, subnet mask in dotted-decimal form, and the full usable IP range together, since they’re all derived from the same CIDR notation.