Reading the results
The prefix length says how many bits at the start of the address name the network; the rest number the hosts. A /24 leaves 8 host bits, so 256 addresses. The first one is the network address and the last is the broadcast address, which leaves 254 for devices. A /31 is used for point-to-point links and has 2 usable addresses, and a /32 is a single host.
Masks and wildcards
The subnet mask is the prefix written as an address, such as 255.255.255.0 for /24. The wildcard mask is its inverse, 0.0.0.255, which is what Cisco access lists and OSPF use. You can type either a prefix or a mask, like 10.0.0.0 255.255.0.0.
Private and special ranges
10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are private and used inside homes and offices. 100.64.0.0/10 is shared space for carrier-grade NAT, 127.0.0.0/8 is loopback and 169.254.0.0/16 shows up when a device couldn't get an address. For IPv6, fd00::/8 is the private range, fe80::/10 is link-local, 2001:db8::/32 is reserved for examples, and a /64 is the normal size for one network.