IPv6 - prefix length
In the context of IPv6, the subnet mask as used in IPv4 does not exist. Instead, IPv6 uses a system called "prefix length" to determine the network and host portions of an IP address.
This prefix length is denoted by a slash ("/") followed by a number, which indicates how many of the first bits in the address specify the network prefix. The rest of the bits (128 minus the prefix length) represent the interface identifier (similar to the host part in IPv4). For example, in the address 2001:db8::/32, the prefix is 2001:db8::, and it's 32 bits long.
IPv6 addressing and subnetting have been made simpler than IPv4. The vast address space makes it possible to allocate networks in a much more straightforward way, often with prefix lengths of /64, which provides a huge number of unique addresses within each subnet.