is expired the process proceeds to step s 308 in which a dhcprequest data packet for example is sent to the dhcp server to renew the lease of the expired ip address in this way the ap merely needs to maintain a small ip address pool in the case of infrequent network layer handoff the pre fetched ip address from the dhcp server for example one or several ip addresses is sufficient so as to avoid occupying excessive resources the ip addresses in the pool can be temporally used by mobile nodes before a formal ip address is obtained from the dhcp server the communication with the temporary ip address only lasts a short duration e g several hundred milliseconds to 1 to 2 seconds to achieve the seamless handoff it will be understood that the steps shown in fig3 and the execution order shown are illustrative and those skilled in the art can modify or adjust the maintenance mechanism of the available ip address pool without departing from the scope of the present invention for example in checking the lease information on the ip addresses in the available ip address pool it is also possible to check whether the lease of any of the reserved ip addresses is expired first and then check the lease of the recycled idle ip addresses fig4 is a flowchart showing in detail the steps of tracing the mobile nodes and recycling the idle ip addresses step s 302 shown in fig3 in a wlan each mobile node is associated with only one ap and the ap can utilize this information to.