RFC 1531 (rfc1531) - Page 3 of 39

Dynamic Host Configuration Protocol

RFC 1531          Dynamic Host Configuration Protocol       October 1993


   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
   host.  In "dynamic allocation", DHCP assigns an IP address to a host
   for a limited period of time (or until the host explicitly
   relinquishes the address).  In "manual allocation", a host's IP
   address is assigned by the network administrator, and DHCP is used
   simply to convey the assigned address to the host.  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
   host to which it was assigned.  Thus, dynamic allocation is
   particularly useful for assigning an address to a host that will be
   connected to the network only temporarily or for sharing a limited
   pool of IP addresses among a group of hosts that do not need
   permanent IP addresses.  Dynamic allocation may also be a good choice
   for assigning an IP address to a new host being permanently connected
   to a network where IP addresses are sufficiently scarce that it is
   important to reclaim them when old hosts 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, 23] and to allow interoperability of existing
   BOOTP clients with DHCP servers.  Using BOOTP relaying agents
   eliminates the necessity of having a DHCP server on each physical
   network segment.






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