The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Dynamic Host Configuration Protocol (DHCP)                      9  DHCP Configuration and Operation

DHCP General Operation and Client Finite State Machine 1 2 3 DHCP Lease Reallocation Process

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.

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 in the 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 name—they 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).

DHCP General Operation and Client Finite State Machine 1 2 3 DHCP Lease Reallocation Process