Dhcp Ip Address Assignment; Ip Address Assignment Policy; Obtaining Ip Addresses Dynamically - H3C S7500 Series Operation Manual

Operation Manual – DHCP
H3C S7500 Series Ethernet Switches

1.2 DHCP IP Address Assignment

1.2.1 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.

1.2.2 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
Packet
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 packet.
4) Acknowledge: Upon receiving the DHCP-REQUEST packet, the DHCP server
returns a DHCP-ACK packet to the DHCP client to confirm the assignment of the
IP address to the client, or returns a DHCP-NAK packet to refuse the assignment
of the IP address to the client. When the client receives the DHCP-ACK packet, it
broadcasts an ARP packet with the assigned IP address as the destination
address to detect the assigned IP address, and uses the IP address only if it
does not receive any response within a specified period.
Format.
1-2
Chapter 1 DHCP Overview
DHCP