Subject: [dhcwg] <draft-ietf-dhc-agent-subnet-selection-03.txt> - Classfull ?

From: Van Aken Dirk <[email-masked]>
Date: Mon, 7 Oct 2002 10:14:18 +0200

Hello Working group,

Could it be that the "Link Selection sub-option for the Relay Agent
Information Option" is a classfull option ?
i.e. According this draft, the format of the sub-option is:

           SubOpt   Len     subnet IP address
          +------+------+------+------+------+------+
          | TBD  |   4  |  a1  |  a2  |  a3  |  a4  |
          +------+------+------+------+------+------+

The sub-option contains a single IP address that is an address con-
tained in a subnet. The value for the subnet address is determined by
taking any IP address on the subnet and ANDing that address with the
subnet mask (i.e.: the network and subnet bits are left alone and the
remaining (address) bits are set to zero).

If I understand the above text correctly, a single (masked) IP address is
added to this sub-option by the relay agent and submitted to the DHCP
server.
The latter will subsequently supply an address within the same subnet.

I wonder though how the DHCP server can derive a subnet from a single
quantity being an IP address. e.g. 20.0.0.0 can be an alias for 20.0.0.0/8
or 20.0.0.0/29.
So my guess is that the only way the DHCP server can derive the subnet form
is based on the leading bits of the address being either class A, B or C.

Am I correct ?

Thanks in advance - Dirk

Dirk Van Aken                              THOMSON multimedia Broadband
Belgium NV
System Architect                           Prins Boudewijnlaan 47,
Tel. : 03/443.65.08                        2650 Edegem
Fax.: 03/443.66.32                         Belgium
[email-masked]

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From: Ted Lemon <[email-masked]>
Date: Tue, 8 Oct 2002 16:31:46 -0500

> quantity being an IP address. e.g. 20.0.0.0 can be an alias for 

The DHCP server is configured with a list of subnets.   It has to be, 
to make address assignment decisions.

This is exactly the same problem that you have if you use giaddr for 
subnet selection.