Enter an IPv4 address and a subnet mask (dotted decimal, e.g. 255.255.255.0) or a prefix (/24) to compute the network address, broadcast address, usable host range, and where the host sits inside its subnet. Pure in-browser math.
Yes. The second field accepts either a dotted-decimal mask such as 255.255.255.0 or a prefix such as /24, and you can also put both in the IP field as 192.168.1.50/24. A mask that is not contiguous, such as 255.0.255.0, is rejected.
It is the position of your IP address inside its own network, counting from zero at the network address. For 192.168.1.50/24 the offset is 50, which tells you how far into the range the host sits.
No. The subnet math runs in your browser and nothing is uploaded or stored, so internal addressing stays on your device.