The Dynamic Host Configuration Protocol (DHCP) provides configuration
parameters to Internet hosts. DHCP consists of two components: a
protocol for delivering host-specific configuration parameters from a
DHCP server to a host and a mechanism for allocation of network
addresses to hosts.
DHCP is built on a client-server model, where designated DHCP server
hosts allocate network addresses and deliver configuration parameters
to dynamically configured hosts. Throughout the remainder of this
document, the term "server" refers to a host providing initialization
parameters through DHCP, and the term "client" refers to a host
requesting initialization parameters from a DHCP server.
A host should not act as a DHCP server unless explicitly configured
to do so by a system administrator. The diversity of hardware and
protocol implementations in the Internet would preclude reliable
operation if random hosts were allowed to respond to DHCP requests.
For example, IP requires the setting of many parameters within the
protocol implementation software. Because IP can be used on many
dissimilar kinds of network hardware, values for those parameters
cannot be guessed or assumed to have correct defaults. Also,
distributed address allocation schemes depend on a polling/defense
mechanism for discovery of addresses that are already in use. IP
hosts may not always be able to defend their network addresses, so
that such a distributed address allocation scheme cannot be
guaranteed to avoid allocation of duplicate network addresses.
DHCP supports three mechanisms for IP address allocation. In
"automatic allocation", DHCP assigns a permanent IP address to a
client. In "dynamic allocation", DHCP assigns an IP address to a
client for a limited period of time (or until the client explicitly
relinquishes the address). In "manual allocation", a client's IP
address is assigned by the network administrator, and DHCP is used
simply to convey the assigned address to the client. A particular
network will use one or more of these mechanisms, depending on the
policies of the network administrator.
Dynamic allocation is the only one of the three mechanisms that
allows automatic reuse of an address that is no longer needed by the
client to which it was assigned. Thus, dynamic allocation is
particularly useful for assigning an address to a client that will be
connected to the network only temporarily or for sharing a limited
pool of IP addresses among a group of clients that do not need
permanent IP addresses. Dynamic allocation may also be a good choice
for assigning an IP address to a new client being permanently
connected to a network where IP addresses are sufficiently scarce
that it is important to reclaim them when old clients are retired.
Manual allocation allows DHCP to be used to eliminate the error-prone
process of manually configuring hosts with IP addresses in
environments where (for whatever reasons) it is desirable to manage
IP address assignment outside of the DHCP mechanisms.
The format of DHCP messages is based on the format of BOOTP messages,
to capture the BOOTP relay agent behavior described as part of the
BOOTP specification [7, 21] and to allow interoperability of existing
BOOTP clients with DHCP servers. Using BOOTP relay agents eliminates
the necessity of having a DHCP server on each physical network
segment.
