Network scanning is a crucial first step in penetration testing. It involves actively probing a target network to discover live hosts, open ports, running services, and potential vulnerabilities. This process helps attackers (and defenders) understand the network's topology and identify potential entry points.

Before any exploitation can occur, a penetration tester needs to know what they are up against. Network scanning provides this foundational intelligence. It answers critical questions like:

• What devices are active on the network?
• What operating systems are they running?
• What services are exposed on these devices?
• Are there any known vulnerabilities associated with these services or versions?

The first step is often identifying which IP addresses on a network are actually active. This is commonly done using techniques like ICMP Echo Requests (traditional pings), ARP requests (on local networks), or TCP/UDP probes to common ports. A successful response indicates a live host.

Once live hosts are identified, port scanning probes specific ports on those hosts to see if services are listening. Ports are numbered endpoints for network communication, with well-known ports assigned to common services (e.g., 80 for HTTP, 443 for HTTPS, 22 for SSH).

Different techniques offer varying levels of stealth, speed, and accuracy. Understanding these is key for effective reconnaissance.

TCP Connect Scan: Attempts to complete the full TCP three-way handshake. If successful, the port is open. This is less stealthy as it leaves a record of connection establishment on the target system.

UDP Scan: Probes UDP ports. UDP is connectionless, so responses are less definitive. An ICMP Port Unreachable message indicates a closed port. No response might mean the port is open or filtered. UDP scans are generally slower and less reliable than TCP scans.

Beyond just knowing a port is open, it's vital to identify the specific service running and its version. This information is critical for finding known exploits. Scanners achieve this by sending specific probes to open ports and analyzing the banner information or protocol responses.

OS fingerprinting attempts to determine the operating system of the target host. This is done by analyzing subtle differences in how various operating systems respond to malformed or specific network packets (e.g., TCP window sizes, IP ID sequence, TTL values). Accurate OS detection can significantly narrow down the types of exploits that might be applicable.

Several powerful tools are available for performing network scans. Proficiency with these tools is essential for any aspiring penetration tester.

Nmap is the de facto standard for network scanning. It's incredibly versatile, supporting a wide range of scanning techniques, OS detection, version detection, and scripting capabilities (Nmap Scripting Engine - NSE) for more advanced tasks.

Masscan is designed for extremely high-speed scanning of large networks. It can scan the entire internet in minutes by using asynchronous transmission and a custom TCP/IP stack.

Tools like hping3 and netcat (nc) can also be used for manual packet crafting and basic scanning tasks, offering a deeper understanding of the underlying network protocols.

Network scanning can be intrusive. Always ensure you have explicit permission before scanning any network that you do not own or manage. Unauthorized scanning can have legal consequences and is unethical. When performing authorized scans, be mindful of network load and potential disruption.