This is a randomly generated 48-bit unique local IPv6 prefix as defined by RFC 4193. It can be used for local IPv6 networking. Everytime the regenerate button is pressed or the page is reloaded a new random prefix will be generated.
Unique local addresses have 48-bit prefixes, leaving 16 bits for local subnetting. Below see the addresses of the first and last subnets.
The IPv6 unique local addresses are used similarly to the IPv4 local adresses e.g. 10.0.0.0/8. Unlike their IPv4 counterpart IPv6 unique local addresses have a 40-bit random part. Therefore if you connect 2 or more unique local networks, by VPN for example, it's very unlikely to ever have address collisions. [+]