TP-Link T2600G-28TS User Manual page 205

identifier and the IP address offered by the server. Based on the server's identifier, servers
are informed whose offer the client has accepted.
4) DHCP acknowledgement: The server selected in the DHCP-REQUEST message commits the
binding for the client to persistent storage and responds with a DHCP-ACK message
containing the configuration parameters for the requesting client. If the selected server is
unable to satisfy the DHCP-REQUEST message (e.g., the requested IP address has been
allocated), the server should respond with a DHCP-NAK message.
5) In Dynamic assignment policy, the DHCP client is assigned an IP address with a lease time
(e.g. 2 hours) from the DHCP server. This IP address will be reclaimed by the DHCP server
when its lease time expires. If the client wants to use the IP address continually, it should
unicast a DHCP-REQUEST message to the server to extend its lease.
After obtaining parameters via DHCP, a host should be able to exchange packets with any other
host in the networks.
The Format of DHCP Message

igure 10-12 The Process of DHCPgives the process of DHCP and Figure 10-13 describes each
F
field in the DHCP message. The numbers in parentheses indicate the size of each field in octets.
The names for the fields given in the figure will be used throughout this document to refer to the
fields in DHCP messages.
1) op：Message type, '1' = BOOT-REQUEST, '2' = BOOT-REPLY.
2) htype：Hardware address type, '1' for ethernet.
3) hlen：Hardware address length, '6' for ethernet.
4) hops：Clients set this field to zero and broadcast the DHCP-REQUEST message , optionally
used by relay-agents when booting via a relay-agent.
5) xid：Transaction ID, a random number chosen by the client, used by the client and server to
associate messages.
6) secs：Filled in by client, seconds elapsed since client started trying to boot.
7) flags：A client that cannot receive unicast IP datagrams until its protocol software has been
configured with an IP address should set the first bit in the 'flags' field to 1 in any
DHCP-DISCOVER or DHCP-REQUEST message that client sends. A client that can receive
Figure 10-13 The Format of DHCP Message
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