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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Dynamic Host Configuration Protocol (DHCP)                      9  DHCP Configuration and Operation

DHCP Configuration and Operation 1 2 DHCP Configuration Parameters, Storage and Communication

DHCP is the newest and most current TCP/IP host configuration protocol. However, as we saw in its overview, it wasn't built “from scratch”—it was designed as an extension of the Boot Protocol. In many ways, DHCP is like “BOOTP with more”, and this can be seen in the basic setup of the protocol and how it works.

Both BOOTP and DHCP are designed based on the common TCP/IP model of client/server operation. In any interaction, one device plays the role of client and the other server. Each has specific responsibilities and must send and receive messages following the protocol described in the DHCP standard. The difference is that where BOOTP involves relatively little work for servers and clients and uses a simple single-message exchange for communication, DHCP requires that both servers and clients do more, and uses several types of message exchanges.

A DHCP server is a network device that has been programmed to provide DHCP services to clients. The server plays a central role in DHCP because DHCP's main function is host configuration, and the server is what configures hosts (clients) that communicate with it. On smaller networks there may be only a single server to support many clients, while larger networks may use multiple servers; regardless of the number of servers, each will usually service many clients.

The following are the key responsibilities of servers in making DHCP work:

• Address Storage and Management: DHCP servers are the owners of the addresses used by all DHCP clients. The server stores the addresses and manages their use, keeping track of which addresses have been allocated and which are still available.
  
  
• Configuration Parameter Storage and Management: DHCP servers also store and maintain other parameters that are intended to be sent to clients when requested. Many of these are important configuration values that specify in detail how a client is to operate.
  
  
• Lease Management: As we saw in the previous section, DHCP servers use leases to dynamically allocate addresses to clients for a limited time. The DHCP server maintains information about each of the leases it has granted to clients, as well as policy information such as lease lengths.
  
  
• Responding To Client Requests: DHCP servers respond to different types of requests from clients to implement the DHCP communication protocol. This includes assigning addresses, conveying configuration parameters and granting, renewing or terminating leases.
  
  
• Providing Administration Services: To support all of the above, the DHCP server includes functionality to allow a human administrator to enter, view, change and analyze addresses, leases, parameters and all other information needed to run DHCP.

DHCP Configuration and Operation 1 2 DHCP Configuration Parameters, Storage and Communication