The nmap command is a versatile tool for network exploration and security auditing. It can scan network entities for open ports and services. When provided with an IP address and port number, it can verify if a given service is running and reachable on that port, which is exactly what is needed to check if a web server is listening on its default port. openssl s_client is typically used to troubleshoot SSL/TLS connections, not to confirm if a service is listening on a specific port. netstat is useful for displaying network connections on the local machine, but it is not designed for testing remote system ports. dig is a tool for querying DNS name servers, which is unrelated to checking for open ports on a remote system.