Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

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Chapter 319
DHCP Shortcuts

DHCP Shortcuts

•  A client may skip the DHCPDISCOVER if it knows the server address that it wants to talk to.
•  Server will respond with a DHCP ACK.
•  If the server responds with a DHCPNAK, the client cannot use the parameters it specified in the DHCPREQUEST:
•  Restart with DHCPDISCOVER
•  DHCPINFORM message can be used by a client to inform a server that it has an IP address but would like some parameters.
•  Reply is in unicast

A simpler approach is used for those clients that were previously serviced by DHCP. The client may skip the DHCP Discover message and instead transmit a DHCPREQUEST message. A server will then respond with a DHCPACK message to fill in the client’s configuration parameters. If any of the consistency checks come back as invalid, the server will respond with a DHCPNAK message, meaning that the client may not reuse the requested IP address. The client will restart the process using the longer method (i.e., starting with DHCPDISCOVER).

The DHCPINFORM message can be used by the client to inform a server that it already has an IP address (through a manual configuration process) but it would like a download of some configuration parameters that may be set for it in the server bindings. The reply from the server is unicast (a major difference from all of the other DHCP messages, which are broadcast) directly back to the client.


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