System information

Network features 417
Operation
DHCP is an extension of BootP. Like BootP, it operates on the
client-server model. However, DHCP has more message types than
BootP. DHCP enables the dynamic allocation of IP addresses to different
clients. It can be used to configure clients by supplying the network
configuration parameters such as gateway or router IP addresses.
In addition, DHCP has a lease system that controls the duration an IP
address is leased to a client. The client can request a specific lease
length, or the administrator can determine the maximum lease length. A
lease can range from one minute to 99 years. When the lease is up or
released by the client, the DHCP server automatically retrieves it and
reassigns it to other clients, if necessary. This is an efficient and accurate
way to configure clients quickly. This saves the administrator from an
otherwise repetitive task. IP addresses can be shared among clients that
do not require permanent IP addresses.
DHCP messages
There are seven different DHCP messages. Each message relays certain
information between the client and server. See Table 99 "DHCP message
types" (page 417).
Table 99
DHCP message types
DHCP Message Types
Description
DHCPDISCOVER Initiates a client request to all servers.
DHCPOFFER Offer from server following client request.
DHCPREQUEST Requests a particular server for services.
DHCPAK Notifies client that requested parameters can
be met.
DHCPNAK Notifies client that requested parameters
cannot be met.
DHCPDECLINE Notifies server that offer is unsatisfactory and
will not be accepted.
DHCPRELEASE Notifies server that IP address is no longer
needed.
DHCP message format
The DHCP message format shown inFigure 76 "DHCP message format"
(page 418) is common to all DHCP messages. Each message consists
of 15 fields: 14 fixed-length fields and one variable length field. The
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