How to scan the most common ports with NMAP?

NMAP (Network Mapper) is a free and open-source tool used for network discover and security auditing. It scans for open ports on a network and has become a standard tool used by network administrators and cybersecurity professionals worldwide.

A common use of NMAP is to scan the most common ports on a machine. Before we start, you’ll need to install NMAP. It’s available for various platforms like Windows, Linux, and macOS, and can be downloaded from the official NMAP site (source: https://nmap.org/download.html).

Once installed, you can scan the most common ports with a basic command in the terminal:

`nmap -F `

Here `-F` stands for fast mode, which scans only the top 100 most common TCP ports (source: https://nmap.org/book/man.html). In place of ``, you need to enter the IP address or hostname of the system you want to scan.

To scan the top ports, replace `-F` with `—top-ports`, followed by the number of ports you want to scan. For example:

`nmap —top-ports 1000 `

This will scan the top 1000 most common TCP ports on the target machine.

NMAP sends specific packets to the targets and analyzes the responses to determine which ports are open. For most port scanning techniques, it sends a packet to the specific port on the target machine and relies on the received response to determine the status of the port. If the port is open, the received response will be different than if the port is closed or filtered (source: https://nmap.org/book/man-port-scanning-techniques.html).

NMAP has many options to granulate your scanning process. For instance, to specify the scanning technique, you can use `-sS` for SYN scan, `-sT` for Connect scan, `-sU` for UDP scan, amongst others.

Remember to use NMAP responsibly. While scanning for open ports on a network is legal in general, unauthorized port scanning may violate laws or rules of your local jurisdiction or Terms of Service of some networks. Always get proper authorization before scanning a network that is not your own. In a professional setting, this usually involves getting explicit written permission from the network’s owner (source: https://tools.ietf.org/html/rfc2828).

Examples used in this answer are for educational purposes only. They provide necessary skills to individuals who are learning network security, IT infrastructure management, or cybersecurity in general.

Sources:

1. Official NMAP site: https://nmap.org/
2. NMAP Man Page: https://nmap.org/book/man.html
3. NMAP Port Scanning Techniques: https://nmap.org/book/man-port-scanning-techniques.html
4. RFC 2828: https://tools.ietf.org/html/rfc2828