Interview question: What is the working process of the ARP protocol?
ARP (Address Resolution Protocol) is a network protocol used to map IP addresses to MAC addresses. It implements the function of converting one type of address into another type of address, and is an essential part of LAN communication. Its working process is as follows:
- Host A needs to send data to another host B in the LAN, but only knows the IP address of the target host.
- Host A first checks whether there is a MAC address corresponding to the IP of the target host in its own ARP cache table, and if so, directly initiates a communication request.
- If there is no required information in the ARP cache table, host A sends an ARP request broadcast packet (Broadcast) to its subnet, which contains information such as the destination IP address and source MAC address.
- All hosts on the LAN will receive the ARP request and accept the target IP address, but only the target host (B) will parse the request and reply to host A with its own MAC address. (Non-target hosts will not respond when receiving ARP packets)
- After host A receives the ARP reply message from the target host, it records the MAC address in the reply packet in its own ARP cache table, and uses this address to generate the frame header of the transmitted data to start data transmission.
To put it simply, the ARP protocol allows each host in the LAN to obtain the IP address of the target host by sending a broadcast request, and then requests the target host for its MAC address. Through this process, host A is able to successfully map the IP address to the MAC address and start network communication.
It should be noted that the mapping relationship between IP addresses and MAC addresses is stored in the ARP cache table. In order to prevent the ARP cache from being modified by malicious attacks, it is usually necessary to enable ARP protection measures, such as static ARP table, dynamic ARP detection and other technologies.