About this tool
What CIDR notation means
CIDR notation writes a block of addresses as an address and a prefix length, such as 192.168.1.0/24. The prefix length is the number of leading bits that every address in the block shares. The remaining bits number the hosts, so a /24 in IPv4 leaves 8 host bits and holds 256 addresses, and each bit added to the prefix halves the block.
A netmask says the same thing in dotted form: 24 one bits followed by 8 zero bits is 255.255.255.0. A wildcard mask is its inverse, 0.0.0.255, and is the form that Cisco IOS access lists and OSPF network statements expect. You can type any of the three and the tool converts between them.
Usable hosts, and the /31 and /32 cases
In IPv4 the first address of a subnet names the network and the last one is the broadcast address, so a subnet has two usable hosts fewer than it has addresses. There are two exceptions. A /31 has only two addresses, and RFC 3021 lets a point-to-point link use both as hosts, with no broadcast address. A /32 is a single host, as used for loopback interfaces and host routes.
IPv6 has no broadcast address, because multicast does that job, so the tool counts every address of an IPv6 block. The first address of an IPv6 subnet is the Subnet-Router anycast address (RFC 4291), which routers answer on. A normal IPv6 subnet is a /64, because stateless address autoconfiguration needs a 64-bit interface identifier.
Address types
Some ranges have a fixed purpose. RFC 1918 reserves 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 for private networks, and 100.64.0.0/10 is the range providers use for carrier-grade NAT. 127.0.0.0/8 is loopback and 169.254.0.0/16 is link-local. In IPv6, fc00::/7 holds the unique local addresses and fe80::/10 the link-local ones. 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 and 2001:db8::/32 exist only for documentation, which is why examples use them.
The classes A, B and C fixed the network size from the first bits of the address until CIDR replaced them in 1993. The tool shows the class for reference only: routing no longer uses it.
How this tool computes it
The address is turned into one integer of 32 bits for IPv4 or 128 bits for IPv6, and the results come from bitwise operations on that integer and the mask. IPv6 addresses are written in the canonical form of RFC 5952: lower case, no leading zeros, and the longest run of zero groups replaced by ::. In the binary row, the vertical bar marks where the network bits end and the host bits begin.