3Com SuperStack 4 Configuration Manual page 82

94 C 5: N
HAPTER ETWORK
DHCP Configuration
Overview of DHCP
P O
ROTOCOL PERATION
Dynamic Host Configuration Protocol (DHCP) offers dynamic IP address
assignment. DHCP works in Client-Server mode. With this protocol, the DHCP
Client can dynamically request configuration information and the DHCP server can
configure the information for the Client.
The DHCP relay serves as conduit between the DHCP Client and the server located
on different subnets. The DHCP packets can be relayed to the destination DHCP
server (or Client) across network segments. The DHCP clients on different
networks can use the same DHCP server. This is economical and convenient for
centralized management.
A typical DHCP application often contains a DHCP server and several clients
(desktop and laptop PCs). See
Figure 22 Typical DHCP Application.
DHCP Client
LAN
DHCP Client
To obtain valid dynamic IP addresses, the DHCP client exchanges different types of
information with the server at different stages. One of the following three
situations may occur:
A DHCP client logs into the network for the first time
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When a DHCP client logs into the network for the first time, its communication
with the DHCP server includes these four stages:
Discovery stage, the stage when the DHCP client looks for the DHCP server.
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The client broadcasts the DHCP_Discover message and only the DHCP
server can respond.
Offer stage, the stage when the DHCP server allocates the IP address. After
■
receiving the DHCP_Discover message from the client, the DHCP server
chooses an IP address still available in the IP address pool for the client, and
sends to the client the DHCP_Offer message containing the leased IP
address and other settings.
Select stage, the stage when the client selects the IP address. If several
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DHCP servers send DHCP_Offer messages to the client, the client only
accepts the first received one and then broadcasts DHCP_Request messages
respectively to those DHCP servers. The message contains the information
of the IP address request from the selected DHCP server.
Acknowledge stage, the stage when the DHCP server acknowledges the IP
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address. When receiving the DHCP_Request message from the client, the
Figure 22
DHCP Client
DHCP Server
DHCP Client