RFC 3011 (rfc3011) - Page 2 of 7

The IPv4 Subnet Selection Option for DHCP

RFC 3011                Subnet Selection Option            November 2000


   To select the subnet on which to allocate an address, the DHCP server
   determines the subnet from which the request originated, and then
   selects an address on the originating subnet or on a subnet that is
   on the same network segment as the originating subnet.  The subnet
   from which the request originates can be determined by:

   o Using the subnet address of the giaddr field in the DHCP packet
     header, or if the giaddr field is zero;

   o Using the subnet address of the local interface on which the DHCP
     server received the packet.

   This memo defines a new DHCP option, the subnet selection option,
   which allows the DHCP client to specify the subnet on which to
   allocate an address.  This option takes precedence over the methods
   that the DHCP server uses to determine the subnet on which to select
   an address.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC 2119].

1.1. Motivational Example

   An example of where this option could be useful is in a device (e.g.:
   a RAS device) that is allocating addresses on behalf of its clients.
   In this case the device would be allocating addresses through DHCP
   and then managing those addresses among its clients.

   In this scenario, the device is connected to a private "internal"
   network on which the DHCP server would be located.  The device is
   also connected to one or more service providing "external" networks
   (i.e.: the networks that the device's clients are connected to).
   Furthermore, the internal network is not IP connected to the external
   networks, although inside the device there is connectivity between
   the internal and external networks (e.g.: though the backplane).

   Recall that the device is allocating addresses for its clients on the
   external networks and that there is no IP connectivity between the
   internal network and the external networks.  The DHCP requests cannot
   originate from the external networks since packets cannot be routed
   between the external network and the internal network.  Thus, the
   DHCP requests must originate from the internal network.  The problem
   with originating the DHCP requests from the internal network is that
   the DHCP server will allocate addresses on the internal network's
   subnet, when what is required are addresses on the external subnets.
   The subnet selection option provides a solution to this problem.




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