Dhcp Address Allocation; Allocation Mechanisms; Dynamic Ip Address Allocation Procedure - 3Com MSR 50 Series Configuration Manual

566 C 32: DHCP O
HAPTER
DHCP Address
Allocation

Allocation Mechanisms

Dynamic IP Address
Allocation Procedure
VERVIEW
relay agent. For information about the DHCP relay agent, refer to
DHCP Relay Agent" on page
DHCP supports three mechanisms for IP address allocation.
Manual allocation: The network administrator assigns an IP address to a client
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like a WWW server, and DHCP conveys the assigned address to the client.
Automatic allocation: DHCP assigns a permanent IP address to a client.
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Dynamic allocation: DHCP assigns an IP address to a client for a limited period
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of time, which is called a lease. Most clients obtain their addresses in this way.
Figure 162 Dynamic IP address allocation process
DHCP client
(1) DHCP-DISCOVER
(2) DHCP-OFFER
(3) DHCP-REQUEST
(4) DHCP-ACK
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
Format" on page 567
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
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address assigned by the server is in use by broadcasting gratuitous ARP. If the
client receives no response within specified time, the client can use this IP
589.
DHCP server
for related information.
"Introduction to
"DHCP Message