IPGadget is a small calculator for verifying IPv4 address information. Given an IP address and network prefix size, IPGadget will show you:
The subnet mask corresponding with the prefix size (eg "/24" = "255.255.255.0")
The network address (ie, where the bits of the host portion are all zeroes)
The broadcast address (ie, where the bits of the host portion are all ones)
The range of valid host addresses for the subnet
IPGadget also highlights suspect addresses where:
The IP address is either the network or broadcast address for the subnet, or is reserved (class E)
The subnet portion of the network prefix is either all zeroes or all ones (classsic mode)
The subnet mask is incorrect for the class of address
IPGadget is particularly useful when designing IPv4 networks. The prefix-size popup menu makes it easy to verify route aggregation or summarization masks, and the stepper quickly recalculates the address field for the next or previous subnet in sequence.
You choose whether to impose classful or classless rules. For example, sometimes it's simpler to skip all-zeroes or all-ones subnets rather than waste time explaining Classless Inter-Domain Routing (CIDR) to people who should know better. Alternatively, if legacy devices in your network preclude you from using CIDR, IPGadget's online help includes an example explaining the classic rules.
IPGadget also provides a handy reference to address-class ranges, the ranges reserved for use in private internets, and the link-local range.