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336 CHAPTER 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:
a IP address in the DHCP Server database, which is statically bound with the
MAC address of the client.
b IP address that was used by the client.
c Address in the requested IP address option contained in the DHCP_Discover
message sent by the client.
d IP address that is first found when searching for the IP addresses available for
allocation in the DHCP address pool in sequence.
e Report the error, if no IP address is available for allocation after going through
the above steps.
■ Applications of DHCP Server
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
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
DHCP server configuration includes:
■ Enable/Disable the DHCP service
■ Create a DHCP address pool
LAN
Router
DHCP Server
Client
Client
Client
Client
Internet
Router
DHCP Relay