How to use it
Type an IPv4 address followed by a prefix length, such as 192.168.1.130/26, or by a dotted mask, such as 192.168.1.130 255.255.255.192. The tool shows the network address (the first address of the block), the broadcast address (the last), the subnet and wildcard masks, the range of addresses you can give to hosts, and how many there are. It also shows the mask in binary, which helps when you learn how prefixes work: a /26 means the first 26 of the 32 bits are the network part. Use Split into smaller subnets to cut the network into 2, 4, 8 or more equal parts and see each one’s range.
The details
In a normal subnet, the first address is the network itself and the last is the broadcast address, so neither can be given to a host: a /24 has 256 addresses but 254 usable hosts. Two special cases are handled as the standards say: a /31 is a point-to-point link with two usable addresses and no broadcast, and a /32 is a single host. Addresses in 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are private and not routed on the internet. The class (A, B, C) is shown for reference only; modern networks use CIDR prefixes and ignore classes.
Questions
Does it do IPv6? Not yet. Why does it reject my mask? A subnet mask must be a block of 1s followed by 0s, so 255.255.0.255 is not valid. Leading zeros in an address (like 010.0.0.1) are refused too, because some systems read them as octal. Is anything sent to a server? No, all the maths is done in your browser. If you work with timestamps in logs, the Timestamp Converter may help.