Network, Computer and Programming Resources

Network - Protocol, DHCP (Dynamic Host

Configuration Protocol) On a TCP/IP-based network, DHCP is used to get information about a client host's (i.e., a network node's) configuration from a DHCP server, which is a specially designated network node. This is useful, for example, in situations where clients are assigned IP addresses dynamically, and where these addresses disappear after a session or after the host relinquishes the address. This is common with Internet Access Providers that assign IP addresses as subscribers connect for a session. The configuration information may not exist until the client requests it. This helps keep down administrative chores. DHCP is similar-and partially equivalent -to the BOOTP protocol, which is used by diskless hosts to get their predefined address when the host connects to the network. DHCP also has similarities with other configuration transmission or retrieval protocols, including RARP (Reverse Address Resolution Protocol), which is used in diskless workstations from Sun Microsystems. However, DHCP goes beyond both of these protocols in that it can handle more than just fixed addresses. In addition to being a protocol, DHCP also provides a mechanism for allocating network addresses. In fact, DHCP provides three mechanisms: n Automatic allocation, in which a permanent IP address is assigned to the host. n Dynamic allocation, in which DHCP assigns a temporary IP address. This mechanism is what distinguishes DHCP from earlier protocols. n Manual allocation, in which the network administrator assigns the address, and DHCP merely transfers the address. A client may request a configuration from any available server by broadcasting a DHCPDISCOVER message to accessible servers. After getting any replies (DHCPOFFER messages), the client selects a server. The selected server's address is included in the DHCPREQUEST message the client sends to all the servers contacted originally. The selected server then begins creating the message with the requested configuration information; the other servers take the Request message as an indication that they have been rejected and that they need not concern themselves with the client any longer. The modified version of DHCP is being developed for use with the new version of the Internet Protocol (IPv6). The revised DHCP will provide dynamic addressing capabilities for the new 128-bit addressing scheme-just as the current version does for nodes in the 32-bit address space. A separate mechanism-stateless autoconfiguration- will be used to provide predefined addresses, such as those used in local links. PRIMARY SOURCES DHCP is discussed in RFCs 1541 (which defines DHCP), 1534, and 1533 (which discuss the relationship between DHCP and BOOTP).