Operation Manual – DHCPH3C S3100 Series Ethernet Switches Chapter 1 DHCP Overview1-31.2.3 Updating IP Address LeaseAfter a DHCP server dynamically assigns an IP address to a DHCP client, the IPaddress keeps valid only within a specified lease time and will be reclaimed by theDHCP server when the lease expires. If the DHCP client wants to use the IP address fora longer time, it must update the IP lease.By default, a DHCP client updates its IP address lease automatically by unicasting aDHCP-REQUEST packet to the DHCP server when half of the lease time elapses. TheDHCP server responds with a DHCP-ACK packet to notify the DHCP client of a new IPlease if the server can assign the same IP address to the client. Otherwise, the DHCPserver responds with a DHCP-NAK packet to notify the DHCP client that the IP addresswill be reclaimed when the lease time expires.If the DHCP client fails to update its IP address lease when half of the lease timeelapses, it will update its IP address lease by broadcasting a DHCP-REQUEST packetto the DHCP servers again when seven-eighths of the lease time elapses. The DHCPserver performs the same operations as those described above.1.3 DHCP Packet FormatDHCP has eight types of packets. They have the same format, but the values of somefields in the packets are different. The DHCP packet format is based on that of theBOOTP packets. The following figure describes the packet format (the number in thebrackets indicates the field length, in bytes):Figure 1-2 DHCP packet formatThe fields are described as follows:z op: Operation types of DHCP packets, 1 for request packets and 2 for responsepackets.z htype, hlen: Hardware address type and length of the DHCP client.
