What tool is used to send a test packet to check if a machine is reachable?

The tool used to send a test packet to check if a machine is reachable is Ping. Designed to send Internet Control Message Protocol (ICMP) Echo Request packets to the target host, Ping measures the time it takes for these packets to reach the destination and return. This helps in determining network connectivity and diagnosing network issues. If the target machine responds, it confirms that the device is reachable over the network.

In contrast, Traceroute and Tracert are utilities that trace the path packets take from one device to another, providing insight into the route and the time taken for each hop along the way. These tools are useful for network troubleshooting but do not specifically test reachability in the same direct manner as Ping.

Ipconfig is primarily used for network configuration management on Windows operating systems. It displays current TCP/IP network configuration values, including the IP address, subnet mask, and default gateway, but it does not send test packets to check reachability.

Thus, Ping's functionality specifically focuses on testing reachability through packet transmission, making it the correct answer to the question.