RADIUS attribute from rfc2865

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NAS-IP-Address

This Attribute indicates the identifying IP Address of the NAS
      which is requesting authentication of the user, and SHOULD be
      unique to the NAS within the scope of the RADIUS server. NAS-IP-
      Address is only used in Access-Request packets.  Either NAS-IP-
      Address or NAS-Identifier MUST be present in an Access-Request
      packet.

      Note that NAS-IP-Address MUST NOT be used to select the shared
      secret used to authenticate the request.  The source IP address of
      the Access-Request packet MUST be used to select the shared
      secret.

   A summary of the NAS-IP-Address 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 (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      4 for NAS-IP-Address.Length

      6

   Address

      The Address field is four octets.

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