A network engineer is asked to verify whether a remote server (10.5.5.100) has port 443 open without using a full vulnerability scanner. They want a quick command-line test.
Which command-line tool or technique can quickly test whether TCP port 443 is open on a remote host from a Windows workstation?
- A.ping 10.5.5.100 -p 443 - ping can test specific ports
- B.tracert 10.5.5.100:443 - tracert supports port specification
- C.telnet 10.5.5.100 443 - the Telnet client attempts a TCP connection to port 443; if it connects (blank screen or TLS handshake data), the port is open; if it fails immediately, the port is closed or filtered
- D.nslookup 10.5.5.100 443 - nslookup tests port availability via DNS
Why C is correct
Telnet client (telnet <ip> <port>) is a classic TCP port connectivity test. If the connection opens (even a blank screen or garbled TLS data), port is open. Connection refused = port closed. No response/timeout = port filtered. On modern Windows, the Telnet client must be enabled (Windows Features). PowerShell alternative: 'Test-NetConnection -ComputerName 10.5.5.100 -Port 443' or 'Test-NetConnection 10.5.5.100 -Port 443 -InformationLevel Detailed'. On Linux: 'nc -zv 10.5.5.100 443' or 'curl -v https://10.5.5.100'. These are Layer 4 connectivity tests.
Know someone studying for Network Fundamentals? Send them this one.