The protocols for resolving addresses and for network diagnostics.
While the Network Layer uses logical IP addresses to route packets between networks, the Data Link Layer requires physical MAC addresses for delivery on the local network segment. The Address Resolution Protocol (ARP) is the bridge between these two. When a host wants to send a packet to another host on the same local network, it knows the destination IP address, but it doesn't know the destination's MAC address. The sending host broadcasts an ARP request packet containing the target IP address to all devices on the LAN. The message essentially asks, 'Who has this IP address? Please tell me your MAC address.' The device that owns that IP address sends back an ARP reply containing its MAC address directly to the original sender. The sender then stores this IP-to-MAC mapping in its ARP cache for future use and sends the IP packet encapsulated in a frame with the correct destination MAC address. The Internet Control Message Protocol (ICMP) is a supporting protocol used by network devices, like routers, to send error messages and operational information. It's a fundamental part of network diagnostics. For example, if a router cannot deliver a packet, it might send an ICMP 'Destination Unreachable' message back to the source. The popular 'ping' utility uses ICMP Echo Request and Echo Reply messages to test connectivity between two hosts. The 'traceroute' utility uses ICMP messages with increasing Time-To-Live (TTL) values to map the path a packet takes through the network.