动态主机设置协议（Dynamic Host Configuration Protocol,
DHCP）是一个局域网的网络协议，使用UDP协议工作，主要有两个用途：
- 给内部网络或网络服务供应商自动分配IP地址给用户
- 给内部网络管理员作为对所有计算机作中央管理的手段
DHCP使用了租约的概念，或称为计算机IP地址的有效期。租用时间是不定的，主要取决于用户在某地联接Internet需要多久，这对于教育行业和其它用户频繁改变的环境是很实用的。通过较短的租期，DHCP能够在一个计算机比可用IP地址多的环境中动态地重新配置网络。DHCP支持为计算机分配静态地址，如需要永久性IP地址的Web服务器。
When a computer or other networked device connects to a network, its DHCP
client software in the operating system 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 the time period for which the allocation
(lease) is valid.
The client broadcasts messages on the network subnet using the destination
address 255.255.255.255 or the specific subnet broadcast
address. A DHCP client may also request its last-known IP address. If the client
remains connected to the same network, the server may grant the request.
Otherwise, it depends whether the server is set up as authoritative or not. An
authoritative server denies the request, causing the client issue a new request.
A non-authoritative server simply ignores the request, leading to an
implementation-dependent timeout for the client expire the request and ask for a
new IP address.
When a DHCP server receives a DHCPDISCOVER message from a client, which is an
IP address lease request, the server reserves an IP address for the client and
makes a lease offer by sending a DHCPOFFER message to the client. This message
contains the client‘s MAC address, the IP address that the server is offering,
the subnet mask, the lease duration, and the IP address of the DHCP server
making the offer.
In response to the DHCP offer, the client replies with a DHCP request, broadcast to the server, requesting the
offered address. A client can receive DHCP offers from multiple servers,
but it will accept only one DHCP
offer. Based on required server identification option in the request and
broadcast messaging, servers are informed
whose offer the client has accepted.[5] When other DHCP servers receive
this message, they withdraw any offers that they might have made to the client
and return the offered address to the pool of available addresses.
When the DHCP server receives the DHCPREQUEST message from the client, the
configuration process enters its final phase. The acknowledgement phase involves
sending a DHCPACK packet to the client. This packet includes the
lease duration and any other configuration information that the client
might have requested. At this point, the IP configuration process is
completed.
After the client obtains an IP address, the client may use the Address
Resolution Protocol (ARP) to prevent
address conflicts caused by overlapping address pools of DHCP
servers.