RFC 2131 (rfc2131) - Page 2 of 45

Dynamic Host Configuration Protocol

RFC 2131          Dynamic Host Configuration Protocol         March 1997


   4.1 Constructing and sending DHCP messages. . . . . . . . . . . . 22
   4.2 DHCP server administrative controls . . . . . . . . . . . . . 25
   4.3 DHCP server behavior. . . . . . . . . . . . . . . . . . . . . 26
   4.4 DHCP client behavior. . . . . . . . . . . . . . . . . . . . . 34
   5.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .42
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .42
   7.  Security Considerations. . . . . . . . . . . . . . . . . . . .43
   8.  Author's Address . . . . . . . . . . . . . . . . . . . . . . .44
   A.  Host Configuration Parameters  . . . . . . . . . . . . . . . .45
List of Figures
   1. Format of a DHCP message . . . . . . . . . . . . . . . . . . .  9
   2. Format of the 'flags' field. . . . . . . . . . . . . . . . . . 11
   3. Timeline diagram of messages exchanged between DHCP client and
      servers when allocating a new network address. . . . . . . . . 15
   4. Timeline diagram of messages exchanged between DHCP client and
      servers when reusing a previously allocated network address. . 18
   5. State-transition diagram for DHCP clients. . . . . . . . . . . 34
List of Tables
   1. Description of fields in a DHCP message. . . . . . . . . . . . 10
   2. DHCP messages. . . . . . . . . . . . . . . . . . . . . . . . . 14
   3. Fields and options used by DHCP servers. . . . . . . . . . . . 28
   4. Client messages from various states. . . . . . . . . . . . . . 33
   5. Fields and options used by DHCP clients. . . . . . . . . . . . 37

1. Introduction

   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



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