Netmask

A Netmask masks the bits of your network for a particular IP. A netmask is a continuous stream of binary 1's from 0 through 32 bits. Here is a few common netmasks

IP: 192.168.0.1
NM: 255.255.255.0

So the network the netmask indicates is 192.168.0.0/24. Netmasks are often written in CIDR notation, in IPv4 and almost exclusively in IPv6. A /64 which is common in delegation in IPv6 holds an immense amount of data, yet it would be wrong to write it as FFFF:FFFF:FFFF:FFFF::, at least it seems wrong.