classify routable and non-routable addresses

[dsheets]

This distinguishes loopback and link-local in the IPv4 case. IPv6 needs investigation.

[dsheets]

RFC 3927 states

   The IPv4 prefix 169.254/16 is registered with the IANA for this
   purpose.  The first 256 and last 256 addresses in the 169.254/16
   prefix are reserved for future use and MUST NOT be selected by a host
   using this dynamic configuration mechanism.

[psafont]

For IPV6 this is relevant:

[2.5.6](https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.6).  Link-Local IPv6 Unicast Addresses

   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         |
   +----------+-------------------------+----------------------------+

   Link-Local addresses are designed to be used for addressing on a
   single link for purposes such as automatic address configuration,
   neighbor discovery, or when no routers are present.

   Routers must not forward any packets with Link-Local source or
   destination addresses to other links.

and I guess the IPv4 link-local addressed translated as well, unlikely as they might be:

[2.5.5.2](https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5.2).  IPv4-Mapped IPv6 Address

   A second type of IPv6 address that holds an embedded IPv4 address is
   defined.  This address type is used to represent the addresses of
   IPv4 nodes as IPv6 addresses.  The format of the "IPv4-mapped IPv6
   address" is as follows:

   |                80 bits               | 16 |      32 bits        |
   +--------------------------------------+--------------------------+
   |0000..............................0000|FFFF|    IPv4 address     |
   +--------------------------------------+----+---------------------+

   See [[RFC4038](https://datatracker.ietf.org/doc/html/rfc4038)] for background on the usage of the "IPv4-mapped IPv6
   address".