A network administrator receives a message that a duplicate IP address is on the network.

The arp command is the best choice for locating a node with a duplicate IP address. Address Resolution Protocol maps IPv4 addresses to MAC addresses on the local network. When a duplicate IP address warning appears, an administrator can use the ARP table to identify the MAC address associated with the conflicting IP address. After the MAC address is known, the administrator can check the switch MAC address table to determine which switch port the device is connected to.

This matches the CompTIA Network+ N10-009 troubleshooting objectives, which include using command-line tools to diagnose network issues. The arp -a command is especially useful when troubleshooting local IPv4 addressing conflicts because it displays known IP-to-MAC mappings.

The ping command can test reachability, but it does not reliably identify the physical node causing the conflict. The ipconfig command displays or refreshes IP settings on the local Windows host, but it does not locate another device on the network. The nslookup command troubleshoots DNS name resolution, not duplicate IP addresses. Therefore, arp is the best command for this situation.