Dynamic Host Configuration
Protocol (DHCP) is a network protocol that enables
a DHCP server to automatically assign an IP
address to an individual computer's TCP/IP stack
software. DHCP assigns a number dynamically from a
defined range of numbers (i.e., a scope)
configured for a given network.
Client
computers configured to use DHCP for IP assignment
do not need to have a statically assigned IP
address. In addition, they generally do not need
to have addresses configured for DNS servers or
WINS servers, as these are also set by the DHCP
server.
DHCP assigns a TCP/IP address when a
system is started. Typically, it works like this:
1. A user turns on a computer with a DHCP
client.
2. The client computer sends a
broadcast request (called a DISCOVER or
DHCPDISCOVER), looking for a DHCP server to
answer.
3. The router directs the DISCOVER
packet to the correct DHCP server.
4. The
server receives the DISCOVER packet. Based on
availability and usage policies set on the server,
the server determines an appropriate address (if
any) to give to the client. The server then
temporarily reserves that address for the client
and sends back to the client an OFFER (or
DHCPOFFER) packet, with that address information.
The server also configures the client's DNS
servers, WINS servers, NTP servers, and sometimes
other services as well.
5. The client sends a
REQUEST (or DHCPREQUEST) packet, letting the
server know that it intends to use the
address.
6. The server sends an ACK (or
DHCPACK) packet, confirming that the client has a
been given a lease on the address for a
server-specified period of time.
When a
computer uses a static IP address, it means that
the computer is manually configured to use a
specific IP address. One problem with static
assignment, which can result from user error or
inattention to detail, occurs when two computers
are configured with the same IP address. This
creates a conflict that results in loss of
service. Using DHCP to dynamically assign IP
addresses minimizes these conflicts.
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