RADIUS attribute from rfc3162

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Login-IPv6-Host

This Attribute indicates the system with which to connect the
      user, when the Login-Service Attribute is included.  It MAY be
      used in Access-Accept packets.  It MAY be used in an Access-
      Request packet as a hint to the server that the NAS would prefer
      to use that host, but the server is not required to honor the
      hint.

   A summary of the Login-IPv6-Host Attribute format is shown below.
   The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Address                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      98 for Login-IPv6-Host

   Length

      18Address

      The Address field is 16 octets in length.  The value
      0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF indicates that the NAS SHOULD
      allow the user to select an address or name to be connected to.
      The value 0 indicates that the NAS SHOULD select a host to connect
      the user to.  Other values indicate the address the NAS SHOULD
      connect the user to.

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