Operation Manual

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the DHCP server with a fixed period of time (e.g., 2 hours), allowing the DHCP server to
reclaim (and then reallocate) IP addresses that are not renewed.
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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.
Figure 9-2 The Process of DHCP
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