A server has recently been configured as a network firewall. After the firewall was configured, users could no longer access a remote server. Which of the following, if properly configured on the…

TCP port 21 is used by FTP for file transfers, which sends credentials and data in plaintext and does not provide remote server access or session security.

TCP port 22 is used by SSH (Secure Shell), which establishes an encrypted and authenticated session for remote command-line administration of servers. SSH is the industry-standard replacement for plaintext Telnet and is the primary protocol for secure remote access on Linux, Unix, and network devices.

TCP port 23 is used by Telnet, which transmits all data including usernames and passwords in unencrypted plaintext, making it unsuitable for secure remote access.

TCP port 25 is used by SMTP for mail transfer between servers and has no role in providing interactive remote access to a server.

TCP port 161 is used by SNMP for network monitoring and device management queries and does not provide any form of interactive remote access or session control.

TCP port 3389 is used by RDP (Remote Desktop Protocol), which provides an encrypted graphical desktop session to remote Windows servers. RDP supports certificate-based server authentication and is the standard method for secure graphical remote administration of Windows Server environments.