local network resources to each client represents one such difficulty. In most environments,
delegating such responsibility to the user is not plausible and, indeed, the solution is to define
the resources in uniform terms, and to automate their assignment.
The DHCP dealt with the issue of assigning an internet address to a client, as well as some
other resources.
DHCP Elements
DHCP is built on a client-server model, where designated DHCP server hosts allocate network
addresses and deliver configuration parameters to DHCP clients. Generally a DHCP server can
allocate configuration parameters to more than one client. Figure 10-6
shows you the model.
Figure 10-6 DHCP model
To meet the different requirements of DHCP clients, DHCP server is always designed to supply
hosts with the configuration parameters in three policies.
1)
Manual Assignment: For the specific DHCP clients (e.g., web server), the configuration
parameters are manually specified by the administrator and are assigned to these clients
via a DHCP server.
2)
Automatic Assignment: The DHCP server must supplies the configuration parameters to
DHCP client with the lease time continued for ever.
3)
Dynamic Assignment: A network administrator assigns a range of IP addresses to DHCP
server, and each client computer on the LAN is configured to request an IP address from
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.
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.
171
