ARP operates at Layer 2 (Data Link layer) of the OSI model, not Layer 3. It is responsible for resolving IP addresses to MAC addresses within a local network segment. When a device on a network needs to communicate with another device using its IP address, it broadcasts an ARP request message to all devices within the same subnet. The device that owns the IP address specified in the ARP request responds with an ARP reply message containing its MAC address. This allows the requesting device to create an ARP cache entry mapping the IP address to the MAC address, facilitating direct communication at Layer 2.
Layer 3 switches typically do not maintain ARP tables. ARP tables are primarily used by devices operating at Layer 2, such as traditional switches and network interface cards (NICs). Layer 3 switches, also known as multilayer switches, operate at both Layer 2 and Layer 3, performing routing functions between different IP subnets. They typically use routing tables to make forwarding decisions based on Layer 3 (IP) addresses rather than ARP tables, which are specific to Layer 2 address resolution.
ARP operates by following a sequence of steps to resolve IP addresses to MAC addresses within a local network. When a device wants to communicate with another device using its IP address, it first checks its ARP cache (if available) for a matching IP-to-MAC address mapping. If the mapping is not found, the device broadcasts an ARP request packet to all devices on the local network, specifying the IP address it wishes to resolve. The device that owns the IP address responds with an ARP reply packet containing its MAC address. The requesting device then updates its ARP cache with the IP-to-MAC address mapping, allowing subsequent communications to occur directly at Layer 2.
ARP is a Layer 2 protocol because it operates within the data link layer of the OSI model. Its primary function is to resolve IP addresses to MAC addresses for communication within a local network segment. ARP messages are encapsulated within Ethernet frames (or other data link layer protocols), making them specific to Layer 2 operations where devices communicate using MAC addresses to exchange data packets within the same physical network segment.