Each network host, workstation or server has its own IP address and subnet mask. When the host communicates with the server, according to its own IP address and subnet mask, as well as the server’s IP address, determine whether the server is in the same network segment as itself:

1. If it is determined to be in the same network segment, it will directly find the other party’s MAC address through the Address Resolution Protocol (ARP), and then fill in the other party’s MAC address into the destination MAC address field of the Ethernet frame header, and send the message out. Two-layer exchange realizes communication;

2. If it is determined to be in a different network segment, the host will automatically use the gateway to communicate. The host first finds the MAC address of the set gateway through ARP, and then fills the MAC address of the gateway (not the MAC address of the opposite host, because the host thinks the communication partner is not the local host) into the destination MAC address field of the Ethernet frame header , Send the message to the gateway, and realize communication through the three-layer routing.