Confusion regarding possible IPv6 Link Local address prefixes

Question

I have trouble understanding how many alternative link local IPv6 address prefixes may exist.

I know that the prefix is fe80::/10 which implies that the first 10 bits must be static as:
       |   10     |
       |  bits    |         6 bits          |          rest              |
       +----------+-------------------------+----------------------------+
       |1111111010|           ??????        |       xxx...xxx            |
       +----------+-------------------------+----------------------------+

Thus, theoretically the next 6 bits regardless of the values will all lead to link local addresses.

The first one should be fe80::, the second fe81::,the last one should be febf::

To be clear, I considered 6 additional bits to complete the total number of bits for the first field (16-bits).

On the other hand, I have seen several resources stating that there can be only four alternative prefixes: (a) fe80::, fe90::, feA0::, and feB0:: which is strange to me because that scheme only considers 2 extra bits and forces the last 4 bits of the first field statically all 0.

To make things really complecated the spec says:

Link-Local addresses are for use on a single link.  Link-Local
   addresses have the following format:

   |   10     |
   |  bits    |         54 bits         |          64 bits           |
   +----------+-------------------------+----------------------------+
   |1111111010|           0             |       interface ID         |
   +----------+-------------------------+----------------------------+

Which pretty much makes the fe80:: the only possible prefix.

What is correct?

Answer 1

There is only one IPv6 Link-Local prefix. It is fe80::/10, but the next 54 bits are required to be 0, so all IPv6 Link-Local networks are fe80::/64.

I have seen several resources stating that there can be only four alternative prefixes: (a) fe80::, fe90::, feA0::, and feB0::

Those resources are incorrect because every one of those "alternative" prefixes must set one or more of the required 0 bits to 1.

that scheme only considers 2 extra bits and forces the last 4 bits of the first field statically all 0.

The last six bits of the first word are required to be 0.

Thus, theoretically the next 6 bits regardless of the values will all lead to link local addresses.

No, the last six bits of the first word are required to be 0.