1-2
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Automatic assignment. The DHCP server assigns IP addresses to DHCP clients. The IP addresses
will be occupied by the DHCP clients permanently.
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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 again at the expiration of
the period. This policy applies to most clients.
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. After the DHCP server receives the
DHCP-DISCOVER packet from the DHCP client, it chooses an unassigned IP address from the
address pool according to the priority order of IP address assignment and then sends the IP
address and other configuration information together in a DHCP-OFFER packet to the DHCP client.
The sending mode is decided by the flag filed in the DHCP-DISCOVER packet, refer to section
DHCP Packet Format
for details.
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: In this phase, the DHCP servers acknowledge the IP address. Upon receiving the
DHCP-REQUEST packet, only the selected 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.
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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 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.
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If there are multiple DHCP servers, IP addresses offered by other DHCP servers are assignable to
other clients.
Updating IP Address Lease
After a DHCP server dynamically assigns an IP address to a DHCP client, the IP address keeps valid
only within a specified lease time and will be reclaimed by the DHCP server when the lease expires. If
the DHCP client wants to use the IP address for a longer time, it must update the IP lease.
