在网络通讯时，源主机的应用程序知道目的主机的IP地址和端口号，却不知道目的主机的硬件地址，而数据包首先是被网卡接收到再去处理上层协议的，如果接收到的数据包的硬件地址与本机不符，则直接丢弃。因此在通讯前必须获得目的主机的硬件地址。ARP协议就起到这个作用。源主机发出ARP请求，询问“IP地址是192.168.0.1的主机的硬件地址是多少”，并将这个请求广播到本地网段（以太网帧首部的硬件地址填FF:FF:FF:FF:FF:FF表示广播），目的主机接收到广播的ARP请求，发现其中的IP地址与本机相符，则发送一个ARP应答数据包给源主机，将自己的硬件地址填写在应答包中(In network communication, the application of the source host know the destination host IP address and port number, but do not know the hardware address of the destination host, the packet is received by the first card to get to the upper layer protocol, if the received data packets and the machine hardware address does not directly discard. Therefore, the hardware address of the destination host must be obtained before communication. The ARP protocol does that. The source host sends a ARP request, ask "IP address is 192.168.0.1 hardware address is the number, and the request is broadcast to the local network (Ethernet frame header hardware address fill FF:FF:FF:FF:FF:FF said broadcast), the destination host receives the broadcast ARP request, consistent with the machine IP address is the send a ARP response packet to the source host, will own hardware address in response to the package)