The Network_ID for generating RFC 7217 stable privacy IPv6 addresses
is by default the UUID of the connection.

Alternatively, prefer "connection.stable-id" as Network_ID to generate
the stable addresses. This allows to configure a set of connections that
all use the same Network_ID for generating stable addresses.

Note that the stable-id and the UUID do no overlap, that is two
connections
    [connection]
    uuid=uuid1
    stable-id=
and
    [connection]
    uuid=uuid2
    stable-id=uuid1
generate distinct addresses.