RADIUS attribute from rfc2865

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NAS-Port

This Attribute indicates the physical port number of the NAS which
      is authenticating the user.  It is only used in Access-Request
      packets.  Note that this is using "port" in its sense of a
      physical connection on the NAS, not in the sense of a TCP or UDP
      port number.  Either NAS-Port or NAS-Port-Type (61) or both SHOULD
      be present in an Access-Request packet, if the NAS differentiates
      among its ports.

   A summary of the NAS-Port 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     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      5 for NAS-Port.

   Length

      6

   Value

      The Value field is four octets.

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