792 CHAPTER 56: DHCP OVERVIEW
DHCP Address
Allocation
Allocation Mechanisms DHCP supports three mechanisms for IP address allocation.
■ Manual allocation: The network administrator assigns an IP address to a client
like a WWW server, and DHCP conveys the assigned address to the client.
■ Automatic allocation: DHCP assigns a permanent IP address to a client.
■ Dynamic allocation: DHCP assigns an IP address to a client for a limited period
of time, which is called a lease. Most clients obtain their addresses in this way.
Dynamic IP Address
Allocation Process
Figure 240 Dynamic IP address allocation process
As shown in the figure above, a DHCP client obtains an IP address from a DHCP
server via four steps:
1 The client broadcasts a DHCP-DISCOVER message to locate a DHCP server.
2 A DHCP server offers configuration parameters such as an IP address to the client
in a DHCP-OFFER message. The sending mode of the DHCP-OFFER is determined
by the flag field in the DHCP-DISCOVER message. Refer to “DHCP Message
Format” on page 793 for related information.
3 If several DHCP servers send offers to the client, the client accepts the first received
offer, and broadcasts it in a DHCP-REQUEST message to formally request the IP
address.
4 All DHCP servers receive the DHCP-REQUEST message, but only the server to
which the client sent a formal request for the offered IP address returns a
DHCP-ACK message to the client, confirming that the IP address has been
allocated to the client, or returns a DHCP-NAK unicast message, denying the IP
address allocation.
n
■ After the client receives the DHCP-ACK message, it will probe whether the IP
address assigned by the server is in use by broadcasting a gratuitous ARP
packet. If the client receives no response within specified time, the client can
use this IP address. Otherwise, the client sends a DHCP-DECLINE message to
the server to request an IP address again.
(1) DHCP-DISCOVER
(2) DHCP-OFFER
(3) DHCP-REQUEST
(4) DHCP-ACK
DHCP client DHCP server
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