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The Dynamic Host Configuration Protocol (DHCP) is a standardized network protocol used on Internet Protocol (IP) networks for dynamically distributing network configuration parameters, such as IP addresses for interfaces and services. With DHCP, computers request IP addresses and networking parameters automatically from a DHCP server, reducing the need for a network administrator or a user to configure these settings manually. ==Overview== Computers use the Dynamic Host Configuration Protocol to request Internet Protocol parameters, such as an IP address, from a network server. The protocol operates based on the client–server model. , modern networks ranging in size from home networks to large campus networks and regional Internet service provider networks commonly use DHCP.〔Peterson LL, Davie BS. (2011). (Computer Networks: A Systems Approach ).〕 Most residential network routers receive a globally unique IP address within the provider network. Within a local network, DHCP assigns a local IP address to devices connected to the local network. When a computer or other networked device connects to a network, the DHCP client software sends a broadcast query requesting necessary information. Any DHCP server on the network may service the request. The DHCP server manages a pool of IP addresses and information about client configuration parameters such as default gateway, domain name, the name servers, and time servers. On receiving a request, the server may respond with specific information for each client, as previously configured by an administrator, or with a specific address and any other information valid for the entire network and for the time period for which the allocation (''lease'') is valid. A client typically queries for this information immediately after booting, and periodically thereafter before the expiration of the information. When a DHCP client refreshes an assignment, it initially requests the same parameter values, but the DHCP server may assign a new address based on the assignment policies set by administrators. On large networks that consist of multiple links, a single DHCP server may service the entire network when aided by DHCP relay agents located on the interconnecting routers. Such agents relay messages between DHCP clients and DHCP servers located on different subnets. Depending on implementation, the DHCP server may have three methods of allocating IP addresses: * ''Dynamic allocation'': a network administrator reserves a range of IP addresses for DHCP, and each DHCP client on the LAN is configured to request an IP address from the DHCP server during network initialization. The request-and-grant process uses a lease concept with a controllable time period, allowing the DHCP server to reclaim (and then reallocate) IP addresses that are not renewed. * ''Automatic allocation'': the DHCP server permanently assigns an IP address to a requesting client from the range defined by the administrator. This is like dynamic allocation, but the DHCP server keeps a table of past IP address assignments, so that it can preferentially assign to a client the same IP address that the client previously had. * ''Manual allocation'': commonly called ''Static allocation'', the DHCP server allocates an IP address based on a preconfigured mapping to each client's MAC address. This feature is variously called ''static DHCP assignment'' by DD-WRT, ''fixed-address'' by the dhcpd documentation, ''address reservation'' by Netgear, ''DHCP reservation'' or ''static DHCP'' by Cisco and Linksys, and ''IP address reservation'' or ''MAC/IP address binding'' by various other router manufacturers. DHCP is used for Internet Protocol version 4 (IPv4), as well as for IPv6. While both versions serve the same purpose, the details of the protocol for IPv4 and IPv6 differ sufficiently that they may be considered separate protocols. For the IPv6 operation, devices may alternatively use stateless address autoconfiguration. IPv6 hosts may also use link-local addressing to achieve operation restricted to the local network link. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dynamic Host Configuration Protocol」の詳細全文を読む スポンサード リンク
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