Linux Nmap for Beginners: A Complete Guide with Examples

Nmap (Network Mapper) is a powerful open-source tool used for network discovery and security auditing. Whether you’re a system administrator, cybersecurity enthusiast, or just curious about how networks work, Nmap is an essential tool to learn.

🔍 What is Nmap?

Nmap scans IP addresses and ports to discover hosts and services on a network. It helps identify open ports, running services, and potential vulnerabilities.

📦 Installing Nmap

$ sudo apt install nmap         # Debian/Ubuntu
$ sudo dnf install nmap         # Red Hat/CentOS
$ brew install nmap             # macOS (Homebrew)

🚀 Basic Nmap Commands

1. Scan a Single IP

$ nmap 192.168.1.1

Explanation: Performs a basic scan on the target IP to check for open ports.

Starting Nmap 7.93 ( https://nmap.org ) at 2025-08-01 15:59 BST
Nmap scan report for 192.168.1.1
Host is up (0.0010s latency).
Not shown: 997 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
443/tcp  open  https

2. Scan a Range of IPs

$ nmap 192.168.1.1-10

Explanation: Scans IPs from 192.168.1.1 to 192.168.1.10.

Starting Nmap 7.93 ( https://nmap.org ) at 2025-08-01 16:00 BST
Nmap scan report for 192.168.1.1
Host is up (0.0012s latency).
PORT   STATE SERVICE
22/tcp open  ssh

Nmap scan report for 192.168.1.2
Host is up (0.0011s latency).
PORT   STATE SERVICE
80/tcp open  http

... (output truncated for brevity)

3. Scan an Entire Subnet

$ nmap 192.168.1.0/24

Explanation: Scans all 256 IPs in the subnet.

Starting Nmap 7.93 ( https://nmap.org ) at 2025-08-01 16:01 BST
Nmap scan report for 192.168.1.10
Host is up (0.0010s latency).
PORT     STATE SERVICE
443/tcp  open  https

4. Detect Operating System

$ sudo nmap -O 192.168.1.1

-O: Enables OS detection (requires root privileges).

Starting Nmap 7.93 ( https://nmap.org ) at 2025-08-01 16:02 BST
Nmap scan report for 192.168.1.1
Host is up (0.0010s latency).
PORT   STATE SERVICE
22/tcp open  ssh

OS details: Linux 5.4 - 5.8
Network Distance: 1 hop

5. Service Version Detection

$ nmap -sV 192.168.1.1

-sV: Detects service versions running on open ports.

Starting Nmap 7.93 ( https://nmap.org ) at 2025-08-01 16:03 BST
Nmap scan report for 192.168.1.1
Host is up (0.0010s latency).
PORT     STATE SERVICE VERSION
22/tcp   open  ssh     OpenSSH 8.2p1 Ubuntu 4ubuntu0.3
80/tcp   open  http    Apache httpd 2.4.41
443/tcp  open  https   nginx 1.18.0

6. Aggressive Scan

$ sudo nmap -A 192.168.1.1

-A: Enables OS detection, version detection, script scanning, and traceroute.

Starting Nmap 7.93 ( https://nmap.org ) at 2025-08-01 16:04 BST
Nmap scan report for 192.168.1.1
Host is up (0.0010s latency).
PORT     STATE SERVICE VERSION
22/tcp   open  ssh     OpenSSH 8.2p1
80/tcp   open  http    Apache httpd 2.4.41
443/tcp  open  https   nginx 1.18.0

OS: Linux 5.4 - 5.8
Traceroute:
192.168.1.1 (1 hop)

7. Scan Specific Ports

$ nmap -p 22,80,443 192.168.1.1

-p: Specifies which ports to scan.

Starting Nmap 7.93 ( https://nmap.org ) at 2025-08-01 16:05 BST
Nmap scan report for 192.168.1.1
Host is up (0.0010s latency).
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
443/tcp  open  https

8. Save Output to File

$ nmap -oN scan_results.txt 192.168.1.1

-oN: Saves output in normal format to a file.

$ cat scan_results.txt
# Nmap 7.93 scan initiated Fri Aug  1 16:06:00 2025 as: nmap -oN scan_results.txt 192.168.1.1
Nmap scan report for 192.168.1.1
Host is up (0.0010s latency).
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
443/tcp  open  https
# Nmap done at Fri Aug  1 16:06:05 2025 -- 1 IP address (1 host up) scanned in 5.00 seconds

🛡️ Best Practices

Always scan your own network or with permission.
Use -T4 for faster scans, but avoid -T5 unless necessary.
Combine -sS (stealth scan) with -Pn to avoid ping checks.

📚 Conclusion

Nmap is a versatile and powerful tool that every beginner in cybersecurity or system administration should master. Start with basic scans and gradually explore advanced options like scripting and automation.

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