RADIUS attribute from rfc2865

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Framed-IP-Netmask

This Attribute indicates the IP netmask to be configured for the
      user when the user is a router to a network.  It MAY be used in
      Access-Accept packets.  It MAY be used in an Access-Request packet
      as a hint by the NAS to the server that it would prefer that
      netmask, but the server is not required to honor the hint.A summary of the Framed-IP-Netmask 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

      9 for Framed-IP-Netmask.

   Length

      6

   Address

      The Address field is four octets specifying the IP netmask of the
      user.

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