Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 313
DHCP

DHCP provides a transport mechanism for passing configuration information (that is located on a server) to requesting hosts on a TCP/IP network. What kind of parameters are used for this information? The parameter information is based on the host requirements RFCs (RFCs 1122, 1123, 1112, etc.) After being supplied with this configuration information, the host should be able to communicate with any other host on the Internet.

It is true that DHCP is based on BOOTP, but it adds much more functionality including the ability to “lease” IP network addresses. DHCP also uses some of the features of BOOTP (relay agent, for forwarding the messages across routers) and is interoperable with existing BOOTP clients (RFC 1534 describes the interoperability functions of BOOTP and DHCP). The DHCP messages are in the exact same format as BOOTP. Refer to the slide. DHCP adds the ability to support “leased” IP addresses and other functions. This allows requesting stations to get their IP addresses from a server and then return them when they are finished. These added functions are described in RFC 2132.

DHCP consists of two parts: a protocol for delivering host-specific configuration parameters, and the ability to allocate IP addresses. It is based on a client/server model in which the host requests information from a server. A host can ask a specific server to supply information to it, or it may simply rely on any server to relay information to it. A server must be preconfigured to handle a specific client’s request, or the server will ignore the request.

The first service provided by DHCP is static storage of network parameters for requesting clients. This information is stored in a database (or table) on a host server. The entries are “keyed.” This means that a unique identifier is used to single out the parameters of a requesting host. This identifier is stated as the “client identifier” (or Chaddr), and the assigned network address, and uniquely identifies the lease between the client and the server for DHCP.