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Chapter 11 DHCP Configuration
11.1 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, 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. Anther 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:
Fig 11-1 DHCP protocol interaction
Explanation:
1. DHCP client broadcasts DHCPDISCOVER packets in the local subnet.
2. On receiving the DHCPDISCOVER packet, DHCP server sends a DHCPOFFER
packet along with IP address and other network parameters to the DHCP client.
3. DHCP client broadcast DHCPREQUEST packet with the information for the DHCP
server it selected after selecting from the DHCPOFFER packets.
4. 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