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Yes, it can be determined if a server is safeguarded by a firewall and displaying a closed status for all ports in nmap. A closed port in nmap means that the port is reachable and responding to the probe packets, but there is no application listening on it. This indicates that there is a device or a firewall between the scanner and the target blocking access to the port. Therefore, if all the ports on a server are displaying a closed status, it is likely that the server is safeguarded by a firewall, and the firewall is blocking access to all the ports.