What is an ARP request?
When a PLC is programmed, or an HMI data tag is configured, they are typically done using the Layer 3 IP address, such as 192.168.1.100. A switched ethernet network however doesn't recognise IP addresses, it only understands Layer 2 addresses, or MAC Addresses. These are the unique 6 byte (00:8B:63:0A:42:44 for example) Media Access Control numbers that every piece of equipment is issued with by the manufacturer.
When a Layer 3 IP Packet is presented to Layer 2 on its way to the physical network, the MAC address relating to the target IP address must be identified, and inserted as the destination MAC address. The Address Resolution Protocol (ARP) is used by a network host to resolve an IP address to a MAC address, allowing the Ethernet frame to be correctly addressed and sent on the wire. There could be a number of such targets in the network, so the network host populates an ARP table, matching MAC and IP addresses for future use.
When the Layer 2 switch receives a data frame and reads the MAC address, it refers to its Learned Address Table to know which port to direct the data frame to.