DHCP servers are not required to respond to every DHCPDISCOVER and DHCPREQUEST message they receive. For example, a network administrator, to retain stringent control over the clients attached to the network, may choose to configure DHCP servers to respond only to clients that have been previously registered through some external mechanism. The DHCP specification describes only the interactions between clients and servers when the clients and servers choose to interact; it is beyond the scope of the DHCP specification to describe all of the administrative controls that system administrators might want to use. Specific DHCP server implementations may incorporate any controls or policies desired by a network administrator.
In some environments, a DHCP server will have to consider the values of the vendor class options included in DHCPDISCOVER or DHCPREQUEST messages when determining the correct parameters for a particular client.
A DHCP server needs to use some unique identifier to associate a client with its lease. The client MAY choose to explicitly provide the identifier through the 'client identifier' option. If the client supplies a 'client identifier', the client MUST use the same 'client identifier' in all subsequent messages, and the server MUST use that identifier to identify the client. If the client does not provide a 'client identifier' option, the server MUST use the contents of the 'chaddr' field to identify the client. It is crucial for a DHCP client to use an identifier unique within the subnet to which the client is attached in the 'client identifier' option. Use of 'chaddr' as the client's unique identifier may cause unexpected results, as that identifier may be associated with a hardware interface that could be moved to a new client. Some sites may choose to use a manufacturer's serial number as the 'client identifier', to avoid unexpected changes in a clients network address due to transfer of hardware interfaces among computers. Sites may also choose to use a DNS name as the 'client identifier', causing address leases to be associated with the DNS name rather than a specific hardware box.
DHCP clients are free to use any strategy in selecting a DHCP server among those from which the client receives a DHCPOFFER message. The client implementation of DHCP SHOULD provide a mechanism for the user to select directly the 'vendor class identifier' values.