What is ARP Protocol?
ARP is an abbreviation for Address Resolution Protocol. It is a protocol used in the local network to resolve and translate IP addresses to MAC addresses. When a network device wants to send data to another network device, it must know the IP address of the other network. However, IP addresses alone are not enough, because IP data packets must be encapsulated into frames before they can be sent over the data link, and the data frame must contain the destination MAC address, so the sender must also obtain the destination MAC address. Each network device needs to obtain the MAC address of the next hop before data encapsulation. The IP address is provided by the network layer, and the MAC address is obtained through the ARP protocol. ARP protocol is an important part of the TCP IP protocol suite, which can obtain the MAC address of the target device through the destination IP address, so as to achieve the reachability of the data link layer.
1. How ARP works:
1) ARP working process:
Before Host A can communicate with Host C, it must first obtain the MAC address of Host C.
2) Send ARP Request:
The MAC address of host C does not exist in the ARP cache table of host A, so host A sends an ARP request to obtain the destination MAC address. The ARP Request packet is encapsulated in an Ethernet frame. The source MAC address in the frame header is the MAC address of host A on the sender side. In this case, since host A does not know the MAC address of host C, the destination MAC address is the broadcast address ff-ff-ff-ff-ff-ff-ff. The ARP Request packet contains the source IP address, destination IP address, source MAC address address, and destination MAC address address, where the destination MAC address is 0. The ARP Request packet is propagated across the entire network and is received by all hosts on the network, including gateways. The gateway will prevent the packet from being sent to other networks.
3) ARP Response:
After receiving the ARP Request packet, all hosts check whether the destination protocol address field matches their own IP address. If there is no match, the host will not respond to the ARP Request packet. If the data match, the host records the source MAC address and source IP address in the ARP packet to its ARP cache table, and then responds with the ARP Reply packet.
Host C responds an ARP Reply packet to Host A. The source protocol address in the ARP Reply packet is the IP address of host C, the destination protocol address is the IP address of host A, the destination MAC address is the MAC address of host A, the source MAC address is its MAC address, and the operation code is set to Reply. ARP reply packets are transmitted through unicast.
2. ARP function:
Its main functions are as follows:
1）.Within the same LAN, allowing the host to dynamically obtain the MAC address of the other host on the network, allowing the two machines to communicate with each other.
2) Provides a way to convert an IP address into a physical address.
3）.Helps to achieve accurate transmission of packets in the network.
To put it simply, the ARP protocol is a bridge for the translation between IP addresses and MAC addresses. In network communication, the source host needs to know the MAC address of the target host to send data, and ARP is a tool to help the host find the MAC address of the target host.