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DHCP and MAC addresses

If somebody accesses the internet via a modem based connection say through an ISP, does that connection/session has a unique MAC address assigned, if yes can it be captured (is that MAC broadcasted in the packets). The folks in American Express security told me that the DHCP server assigns a unique identifier (similar to a MAC) and binds it to that userid?. I am getting conflicting answers, some says it can be captured for dial up modem connection? I am not sure about this, hence would like to check this with gurus and get an independent opinion.

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Here's how the whole process works . Hope this will clear all your doubts.
In a dial up session , the DHCP server defines a client identifier, which is used for client identification. If this option is not used, the client is identified by its MAC address.
DHCP provides persistent storage of network parameters for network clients. The model of DHCP persistent storage is that the DHCP service stores a key-value entry for each client, where the key is some unique identifier such as an IP subnet number and a unique identifier within the subnet, and the value contains the configuration parameters for this particular client.
The only time anything is braodcasted by a client is during the DHCP network address allocation process, as a DHCPREQUEST message which includes the "server identifier'' option .The server selected in the DHCPREQUEST message commits the binding for the client to persistent storage and responds with a DHCPACK message containing the configuration parameters for the requesting client. The combination of client hardware and assigned network address constitute a unique identifier for the client's lease and are used by both the client and server to identify a lease referred to in any DHCP messages.
During this configuration committment process nothing is broadcasted.

This was first published in February 2002

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