TP-Link T2500-28TC User Manual page 145

The Process of DHCP

DHCP uses UDP as its transport protocol. DHCP messages from a client to a server are sent to
the 'DHCP server' port (67), and DHCP messages from a server to a client are sent to the 'DHCP
client' port (68). DHCP clients and servers both construct DHCP messages by filling in fields in the
fixed format section of the message and appending tagged data items in the variable length option
area. The process is shown as follows.
1) DHCP discover: the client broadcasts messages on the physical subnet to discover available
DHCP servers in the LAN. Network administrators can configure a local router (e.g. a relay
agent) to forward DHCP-DISCOVER messages to a DHCP server in a different subnet.
2) DHCP offer: Each server who received the DHCP-DISCOVER message may respond a
DHCP-OFFER message that includes configuration parameters (in the example below, IP
address) to the client. The server unicast the DHCP-OFFER message to the client (using the
DHCP/BOOTP relay agent if necessary) if possible, or may broadcast the message to a
broadcast address on the client's subnet.
3) DHCP request: A client can receive DHCP offers from multiple servers, but it will accept only
one DHCP-OFFER and broadcast a DHCP-REQUEST message which includes the server's
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

DHCP model gives the process of DHCP and Figure 9-3 describes each field in the
Figure 9-1
DHCP message. The numbers in parentheses indicate the size of each field in octets. The names
igure 9-2 The Process of DHCP
F
135