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DHCP Lease Allocation Process
(Page 1 of 3)
To implement DHCP, an administrator
must first set up a DHCP server and provide it with configuration parameters
and policy information: IP address ranges, lease length specifications,
and configuration data that DHCP hosts will need to be delivered to
them. Host devices can then have their DHCP client software enabled,
but nothing will happen until the client initiates communication with
the server. When a DHCP client starts up for the first time, or when
it has no current DHCP lease, it will be in an initial state where it
doesn't have an address and needs to acquire one. It will do so by initiating
the process of lease allocation.
Notes on Lease Communication Descriptions
Before I begin, some notes about
this description, which also apply to subsequent topics in this section
on DHCP lease communications:
- Addressing: DHCP assumes that clients
will normally broadcast messages, since they don't know the address
of servers when they initiate contact, but that servers will send replies
back unicast to the client. This can be done even before the client
has an IP address, by sending the message at the link layer. Some clients
don't support this and require that messages to them be broadcast instead.
The details of how messages are created and addressed are discussed
section on DHCP messaging.
- DHCP Message Fields and Options: DHCP
uses many of the same basic fields as BOOTP, but much of the extra information
the protocol requires is carried in DHCP options. Obviously,
some of these options aren't really optional, despite the
namethey are needed for the basic function of DHCP. An obvious
example would be the DHCP Message Type option, which is what
specifies the message type itself. Again, you can find a full description
of all DHCP fields and options in the
section on DHCP messaging.
- DHCP Relay Agents: I have assumed that
no relay agents are in use here; see the
topic on DHCP/BOOTP relay agents for more
on how they change the allocation process (and other processes).
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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