IPv6 - Site Local addresses
Site local IPv6addresses, defined as fec0::/10 were originally reserved for use with a single “site”. However, there was an insufficient definition of the term “site” and this led to confusion over the governing routing rules.
For this reason, this definition was deprecated and replaced with fc00::/7 for use as unique local addresses (ULAs), which are essentially the counterparts of IPv4 private addresses. The first bit following the prefix, if set, indicates that the address is locally assigned. This splits the address block into two equally sized halves, fc00::/8 and fd00::/8.
Currently, fc00::/8 is undefined and reserved for future use while only the fd00::/8 is currently used as ULAs.