IPv6 Prefix Primer
IPv6 format. IPv6 addresses are 128-bits in length. Although IPv6 addresses are 4 times as long as IPv4 addresses, IPv4 addresses can still be written in IPv6 notation. Two colon characters (::) can also be strung together to shorten notation and eliminate groups of zeroes. Unspecified Address - ::/128 An IPv6 address with all 128 bits set to zero is called the unspedified adddress (which correspond to 0.0.0.0 in IPv4). This address should not be assigned to any host and it should only be used as the source address by initializing host before it has learned of its own address. Catalyst 3750 Software Configuration Guide, Release 12.2
IPv6 is going to replace IPv4. The simple reason is that IPv4 address space is running out. The world has reached the point where there are not enough 32 bit addresses to link every device which wants to connect to the Internet. IPv6 uses 128 bits. Much larger. Much much larger. IPv6 allows almost 8*10 28 times as many addresses as IPv4
An IPv6 subnet mask is written in hexadecimal, but let's start by explaining that IPv6 uses 128 binary digits for each IP address, as opposed to IPv4's 32 binary digits, and those 128 binary digits are divided into eight 16-bit words (8 x 16 = 128), like this:
Subnet masks (IPv4) and prefixes (IPv6) - IBM
Jul 07, 2019 IPv6 Prefix Primer