3Com WX3000 operation manual Obtaining IP Addresses Dynamically

z

z

z

Manual assignment. The administrator configures static IP-to-MAC bindings for some special clients, such as a 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 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 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.

z

z

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.

If there are multiple DHCP servers, IP addresses offered by other DHCP servers are assignable to other clients.

1-2