How to scan a specific port using NMAP?

NMAP is a widely recognized and used tool for network exploration and security auditing. The US National Institute of Standards and Technology (NIST) describes it as a tool for network discovery and security auditing in the NIST National Vulnerability Database (https://nvd.nist.gov/ncp/repository).

Scanning a specific port requires a general understanding of how network ports work. Every internet-based service, such as an HTTP server or an FTP server, communicates through a specific port, a digital “doorway” of sorts. There are 65,535 ports available for use in TCP/IP.

To scan a specific port using NMAP, you need to use the -p option followed by the port number. In the command syntax, for instance, if you want to check the status of port 22 (commonly used by SSH servers), it would be as follows:

`nmap -p 22 `

Here, replace `` with the IP of the target system you wish to scan. What this command does is send data packets to port 22 on the target system and analyze the response to learn about the service running on that port.

A response from NMAP might look like this:

\`\`\`PORT STATE SERVICE
22/tcp open ssh\`\`\`

This shows that port 22 has the SSH service running and is open to connections (source: NMAP.org manual https://nmap.org/book/man-port-scanning.html).

Remember, however, that excessive scanning, especially over the internet, may be considered intrusive and unlawful. The Internet Systems Consortium (ISC) cautions that unauthorized scanning can be interpreted as a prelude to an attack, potentially triggering legal ramifications. Always seek permission before conducting any scan (source: ISC, Security Information/Event Management https://www.icsalabs.com/technology-program/siem).

Lastly, I’d like to provide a word of caution about relying on free tools like NMAP for serious, professional use. That’s because these tools, while valuable, can’t provide complete network protection listings. For example, Cisco, in a white paper on network security, notes that professional use requires more sophisticated tools for comprehensive security management. It’s recommended to use NMAP as a part of bigger security auditing toolset (source: Cisco’s white paper, “A Framework for Network Security Overviews” https://www.cisco.com/c/dam/en_us/about/doing_business/legal/global_export_trade/docs/network.pdf).