Enter an IPv4 address and mask to inspect its subnet: network and broadcast address, usable host range and address count. The other calculators on the page convert between prefix lengths, dotted masks, wildcard masks and binary, hexadecimal or integer notation.
Runs locally in your browserEnter 192.168.0.1 with prefix 24 and the first panel prints mask 255.255.255.0, network 192.168.0.0, first host 192.168.0.1, last host 192.168.0.254, broadcast 192.168.0.255 and 254 usable addresses.
Nine more panels share the page: prefix to dotted mask, mask to prefix, wildcard masks, bit inversion, a classful subnet list, required-host sizing, and dotted, binary, hexadecimal and integer notation. They all run in this browser tab and the page issues no request while you calculate.
The mask marks the network bits. Prefix 24 is 255.255.255.0, so the first three octets address the network and the last one addresses hosts: 192.168.0.1 AND 255.255.255.0 gives the network 192.168.0.0, and OR with the inverted mask gives the broadcast 192.168.0.255. The usable range is that block without its two ends, 192.168.0.1 to 192.168.0.254, which is 2^(32−24)−2 = 254 addresses.
The block grows as the prefix shrinks: /8 on 10.0.0.5 covers 10.0.0.0 to 10.255.255.255 with 16,777,214 usable addresses, and /12 on 172.16.5.9 ends at 172.31.255.255 with 1,048,574. Prefixes 0 to 30 follow the 2^(32−prefix)−2 rule; a /31 is a point-to-point link whose two addresses are both usable (RFC 3021), and a /32 is one address with no separate network or broadcast address.
A subnet mask is a run of 1 bits followed by a run of 0 bits, so 255.255.255.0 and 255.240.0.0 are masks and 255.0.255.0 is not. The page refuses a dotted mask that breaks the rule instead of calculating with it; the earlier script accepted 255.0.255.0 and answered 192.168.0.1 with network 192.0.0.0 and broadcast 192.255.0.255.
The same rule applies to the wildcard mask and to mask-to-prefix conversion. Input that is not a whole number in range is refused with a sentence in the language of the page rather than a wrapped value: 256.1.1.1 is no longer read as 255.1.1.1, prefix 33 is an error instead of a full mask, and a required-host figure must be between 1 and 4,294,967,294.
The classful panel works from the class of the address (1–127 class A, 128–191 class B, 192–223 class C; 0.0.0.0 and anything from 224.0.0.0 up is refused) or from a network type you pick, and asks for either a subnet count or a hosts-per-subnet figure. Eight subnets inside the class C address 192.168.0.1 give mask 255.255.255.224, prefix /27, eight subnets and 30 usable hosts each; asking for 1024 hosts instead gives /21 and covers eight class C networks, which the panel reports as supernetting.
Network List prints every subnet of that network with its first and last host and its broadcast address, and stops after 256 rows with a "…" row. A request that cannot fit the class is reported as an error instead of being clamped: the earlier script turned 256 requested subnets in a class C address into 128, and could print host counts such as 5592405.333333333.