Manual

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Chapter 23 DHCP Configuration
23.1 Introduction to DHCP
DHCP [RFC2131] is the acronym for Dynamic Host Configuration Protocol. It is a protocol that
assigns IP address dynamically from the address pool as well as other network configuration
parameters such as default gateway, DNS server, and default route and host image file
position within the network. DHCP is the enhanced version of BOOTP. It is a mainstream
technology that can not only provide boot information for diskless workstations, but can also
release the administrators from manual recording of IP allocation and reduce user effort and
cost on configuration. Another benefit of DHCP is it can partially ease the pressure on IP
demands, when the user of an IP leaves the network that IP can be assigned to another user.
DHCP is a client-server protocol, the DHCP client requests the network address and
configuration parameters from the DHCP server; the server provides the network address and
configuration parameters for the clients; if DHCP server and clients are located in different
subnets, DHCP relay is required for DHCP packets to be transferred between the DHCP client
and DHCP server. The implementation of DHCP is shown below:
DHCP protocol interaction
Explanation:
DHCP client broadcasts DHCPDISCOVER packets in the local subnet.
On receiving the DHCPDISCOVER packet, DHCP server sends a DHCPOFFER packet along
with IP address and other network parameters to the DHCP client.
DHCP client broadcast DHCPREQUEST packet with the information for the DHCP server it
selected after selecting from the DHCPOFFER packets.
The DHCP server selected by the client sends a DHCPACK packet and the client gets an IP
address and other network configuration parameters.
The above four steps finish a Dynamic host configuration assignment process. However, if the
DHCP server and the DHCP client are not in the same network, the server will not receive the
DHCP broadcast packets sent by the client, therefore no DHCP packets will be sent to the
client by the server. In this case, a DHCP relay is required to forward such DHCP packets so
that the DHCP packets exchange can be completed between the DHCP client and server.
DHCP CLIENT
DHCP
SERVER
Discover
Offer
Request
Ack