3Com 10014299 manual Dhcp Server Configuration

336CHAPTER 21: CONFIGURING IP APPLICATION

will take back the IP address. If the DHCP client wants to continue to use this address, it should renew the IP leasing contract such as extending the leasing contract of the IP address.

■In practice, the DHCP client will automatically send the message for renewing the IP leasing contract to the DHCP server when the DHCP client starts up or half of the valid period of the IP leasing contract has expired. To renew the IP leasing contract, the DHCP client will send a DHCP_Discover message to the DHCP server. If the IP address is valid, the DHCP server will send back a DHCP_ACK message to notify the DHCP client that it has obtained a new IP leasing contract. In addition, the ipconfig /renew command can be used on the user PC (that is, the DHCP client) to renew its IP leasing contract.

■Priority sequence in which the DHCP server allocates IP address to the client The DHCP server will select an IP address for a client in the following order:

aIP address in the DHCP Server database, which is statically bound with the MAC address of the client.

bIP address that was used by the client.

cAddress in the requested IP address option contained in the DHCP_Discover message sent by the client.

dIP address that is first found when searching for the IP addresses available for allocation in the DHCP address pool in sequence.

eReport the error, if no IP address is available for allocation after going through the above steps.

■Applications of DHCP Server

DHCP Server Configuration

In practice, to improve the serving efficiency of DHCP servers, a DHCP server will probably be used to serve the clients beyond the LAN. Normally, routers have been added with the function of DHCP relay proxy (that is, providing the across-segment transmission of DHCP packets). Clients in the LAN can communicate with the DHCP servers in other subnetworks through the DHCP relay proxy and finally obtain legal IP addresses.

Figure 125 Integrated networking application of DHCP server and DHCP relay

Client	Client	DHCP Server
		DHCP Relay
		Router
	LAN	Internet
		Router

ClientClient

Normally, DHCP relay proxy can either be a host or a router as long as the service program of DHCP relay proxy is enabled.

DHCP server configuration includes:

■Enable/Disable the DHCP service

■Create a DHCP address pool