1-2
z 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.
z Automatic allocation: DHCP assigns a permanent IP address to a client.
z Dynamic allocation: DHCP assigns an IP address to a client for a limited period of time, which is
called a lease. Most DHCP clients obtain their addresses in this way.
Dynamic IP Address Allocation Process
Figure 1-2 Dynamic IP address allocation process
As shown in
Figure 1-2, 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.
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 request the IP address formally.
4) All DHCP servers receive the DHCP-REQUEST message, but only the server from which the client
accepts the offered IP address returns a DHCP-ACK message to the client, confirming that the IP
address has been allocated to the client, or a DHCP-NAK unicast message, denying the IP
address allocation.
z 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 the specified time, the client can use this IP address. Otherwise, the client sends a
DHCP-DECLINE message to the server and requests an IP address again.
z IP addresses offered by other DHCP servers are still assignable to other clients.
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