3Com 10014299 manual Dhcp client logs into the network again

DHCP Server Configuration 335

■DHCP client logins the network for the first time

If it is the first time for a DHCP client to login to the network, it will establish a connection with the DHCP server through four stages:

■Discovering stage. This is the stage when the DHCP client searches the DHCP servers. The DHCP client broadcasts a DHCP_Discover message to search the DHCP servers, and every host installed with the TCP/IP suite on the network will receive this type of broadcast message but only the DHCP servers respond to it.

■Offering stage. This is the stage when the DHCP servers offer IP addresses. Upon receiving the client DHCP_Discover message, the DHCP servers select an unallocated IP address from the IP address pools for the DHCP client, and send the DHCP_Offer message containing leased IP address and other settings to the DHCP client.

■Selecting stage. This is the stage when the DHCP client selects the IP address offered by a certain DHCP server. If multiple DHCP servers send the DHCP_Offer messages to it, the DHCP client will accept only the first received DHCP_Offer message. Then, it will respond with a DHCP_Request message by means of broadcasting. This message requests the selected DHCP server for an IP address.

■Acknowledgement stage. This is the stage when the DHCP server acknowledges the offered IP address. Upon receiving the DHCP_Request message from the DHCP client, the DHCP server sends back a DHCP_ACK message containing the offered IP address and other settings to the DHCP client, advising that the offered IP address can be used. Then, the DHCP client will bind its TCP/IP suite with the network card. Except the server selected by the DHCP client, other DHCP servers will use their unallocated IP addresses for the applications of other clients for IP addresses.

■DHCP client logs into the network again:

■Once the DHCP client logs into the network correctly, it merely needs to send the DHCP_Request message containing the IP address allocated previously (there is no need to send a DHCP_Discover message once again).

■Upon receiving the DHCP_Request message, the DHCP server will allow the DHCP client to continue to use the original IP address and will return the DHCP_ACK message.

■If the IP address can not be allocated to the DHCP client again (in this case, the IP address has been allocated to another DHCP client), the DHCP server will return a DHCP_NAK message. When the DHCP client receives the DHCP_NAK message, it needs to send a DHCP_Discover message to request a new IP address.

In addition, you can use the ipconfig /release_all command in the command line on the user PC (that is, the DHCP client) to release the IP address. In this case, the user PC sends a DHCP_Release message to the DHCP server. Then, you can use the ipconfig /renew_all command on the user PC to apply for a new IP address. In this case, the user PC sends a DHCP_Discover message to the DHCP server.

■DHCP client extends the valid period for leasing the IP address:

■The dynamic IP address of the DHCP client allocated by the DHCP server usually has a certain valid leasing period. Upon the expiry, the DHCP server