PLANET WGSW-50040 - Manual (Page 139)

18-1
Chapter 18 DHCP Configuration
18.1 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:
Figure 18-1 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 DHCP packets so that the DHCP packets exchange can be completed
between the DHCP client and server.
Switch can act as both a DHCP server and a DHCP relay. DHCP server supports not only dynamic IP address
assignment, but also manual IP address binding (i.e. specify a specific IP address to a specified MAC address
or specified device ID over a long period. The differences and relations between dynamic IP address
allocation and manual IP address binding are: 1) IP address obtained dynamically can be different every time;
DHCP CLIENT
DHCP SERVER
Discover
Offer
Request
Ack