Free tool
IP Subnet Calculator
Turn an IPv4 address and prefix into the network address, broadcast, usable host range, mask, wildcard and host count.
Reading the result
The network address identifies the subnet itself and the broadcast address reaches everything on it, so neither can be given to a device. That is why a /24 holds 256 addresses but only 254 usable ones. A /31 is the exception, used for point to point links where both addresses are valid, and a /32 is a single host route.
The wildcard mask is the inverse of the subnet mask and is what access control lists on Cisco equipment expect, which is a common place to lose an afternoon.
Private ranges (10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16) are not routable on the public internet and are flagged in the results. Plan the addressing before pulling cable, not after.