Qtech QSW-2900-24T-AC [107/209] Dhcp ip address assignment

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Figure 1-1 Typical DHCP application

6.3 DHCP IP Address Assignment

6.3.1 IP Address Assignment Policy

Currently, DHCP provides the following three IP address assignment policies to meet the requirements of

different clients:

· Manual assignment. The administrator statically binds IP addresses to few clients with special uses

(such as WWW server). Then the DHCP server assigns these fixed IP addresses to the clients.

· Automatic assignment. The DHCP server assigns IP addresses to DHCP clients. The IP addresses will

be occupied by the DHCP clients permanently.

· Dynamic assignment. The DHCP server assigns IP addresses to DHCP clients for predetermined

period of time. In this case, a DHCP client must apply for an IP address at the expiration of the period.

This policy applies to most clients.

6.3.2 Obtaining IP Addresses Dynamically

A DHCP client undergoes the following four phases to dynamically obtain an IP address from a DHCP server:

1) Discover: In this phase, the DHCP client tries to find a DHCP server by broadcasting a DHCP-DISCOVER

packet.

2) Offer: In this phase, the DHCP server offers an IP address. Each DHCP server that receives the

DHCP-DISCOVER packet chooses an unassigned IP address from the address pool based on the IP address

assignment policy and then sends a DHCP-OFFER packet (which carries the IP address and other configuration

information) to the DHCP client. The transmission mode depends on the flag field in the DHCP-DISCOVER packet.

For details, see section DHCP Packet Format.

3) Select: In this phase, the DHCP client selects an IP address. If more than one DHCP server sends

DHCP-OFFER packets to the DHCP client, the DHCP client only accepts the DHCP-OFFER packet that first arrives,

and then broadcasts a DHCP-REQUEST packet containing the assigned IP address carried in the DHCP-OFFER