Open ports in your network hardware can represent significant vulnerability points if the services associated with them are improperly configured or lack necessary updates. Many organizations inadvertently expose sensitive resources through these connections, thereby increasing their susceptibility to ransomware attacks, supply chain compromises, and data breaches.
Fortunately, there are freely available tools designed to detect all open ports within your ecosystem. This allows you to evaluate the criticality of each exposure and take necessary steps to secure your network.
This article will explore five of the Best Free Port Scanning Tools that you can start utilizing today to identify open ports within your environment and enhance your cybersecurity posture.
Understanding Open Port Scanning
Port scanning is a crucial process in network security that involves analyzing the security status of network ports. It encompasses identifying which ports are open and sending data packets to specific ports on a host to uncover potential vulnerabilities in data reception.
Regular network reconnaissance through port scanning is essential for proactively identifying and remediating vulnerabilities before they can be exploited by cybercriminals.
Cybercriminals also employ port scanners to gather vulnerability intelligence about potential targets before initiating cyberattacks. The widespread availability of these tools means you must assume malicious actors are actively scanning your open ports. Therefore, relying solely on open port awareness is insufficient for robust network security.
Free open port scanners are valuable for network administrators to understand the level of network visibility accessible to potential attackers. However, for comprehensive security, these free tools should be complemented with robust, additional security solutions.
TCP port scanners are also instrumental for penetration testers, enabling them to pinpoint specific ports that accept data, allowing for focused security measures to prevent compromise.
Open vs. Closed Ports: What’s the Security Stance?
An open port allows data packets to freely enter and exit your local network. Conversely, a closed port blocks all traffic associated with it, preventing external access to the local network through that specific pathway.
The status of a port, whether open or closed, is typically managed through a network firewall.
While closing all network ports might seem like a straightforward approach to prevent exploitation, it would also disrupt the functionality of essential networked devices required for business operations.
For instance, enabling remote computer connections to a business network necessitates keeping port 3389 open and forwarding to each remote device. Disabling this port would impede the remote workforce model that is vital in today’s business landscape.
Similarly, email communication relies on SMTP ports (ports 25, 465, and 587), which must remain operational for email services to function.
Open ports are not inherently dangerous. The security risks arise from the external services communicating through them. If a device or vendor connected to a network port exhibits a weak security posture, the corresponding open port becomes a potential threat and should be either closed or filtered.
When the security of connected services is reliable, their associated ports can safely remain open.
Common Network Port Numbers: A Quick Guide
Each network port is designated to handle a specific type of network traffic and is assigned a unique number for easy identification.
There are two primary types of network ports:
- TCP (Transmission Control Protocol) – Emphasizes reliable data delivery.
- UDP (User Datagram Protocol) – Prioritizes speed over guaranteed delivery.
TCP vs. UDP: Key Differences
TCP employs a handshake protocol to establish connections and includes error checking for each data packet, ensuring data integrity. UDP, in contrast, omits verification, error checks, and handshakes, making it faster but less reliable than TCP.
Port numbers range from 0 to 65,535, totaling 65,536 ports, which can be TCP, UDP, or both. To simplify network management, port numbers are categorized by relevance:
- Ports 0-1023 (Well-known Ports): Primarily reserved for common internet services.
- Ports 1024-49151 (Registered Ports): Designated for exclusive use by registered software corporations and applications.
- Ports 49152-65,536 (Dynamic or Private Ports): Available for use by any application or service.
For a more detailed overview of common network port numbers, refer to this resource.
How Open Port Scanning Operates
Port scanners function by sending either TCP or UDP data packets to a target port to request a status report.
The scanner can receive three possible responses:
- Open: The target responds with a packet indicating it is “listening,” meaning the port is open and actively accepting connections.
- Closed: The target replies with a message indicating the port is in use but not accepting connections.
- Filtered: The target does not respond at all. This usually signifies that a firewall or filtering system has blocked the data request packet. For enhanced security, closed ports should be secured behind a firewall.
There are five primary types of port scanning techniques:
Ping Scan: Basic Host Discovery
This is the most fundamental type of port scan. It involves sending a burst of Internet Control Message Protocol (ICMP) requests to multiple web servers.
An ICMP reply confirms that data packets can reach the targeted IP address without errors, verifying that the target is “alive” and reachable.
A ping scan typically precedes a full port scan to determine if a computer is present at the target address.
To prevent external threats from discovering your network assets through ping scans, ICMP should be disabled for external traffic via firewall or router settings. However, ICMP should remain enabled for internal traffic to facilitate network troubleshooting.
TCP Half-Open Scan (SYN Scan): Stealth Reconnaissance
This is a widely used port scanning technique, often referred to as a SYN scan.
A standard TCP transaction initiates with a three-step handshake:
- A SYN packet is sent to request a connection.
- The target responds with a SYN-ACK packet.
- An ACK packet is sent back to confirm the response and establish the connection.
A TCP half-open port scan intentionally omits sending the final ACK packet, thus not completing the handshake.
By interrupting the handshake, only a SYN-ACK response is possible. This response indicates an open port. No response suggests a filtered port.
Because it doesn’t finalize the TCP handshake, this scan type is difficult to detect and highly scalable. Running TCP half-open scans requires elevated user privileges.
TCP Connect Scan: Full Connection Scan
A TCP connect port scan takes the process a step further than the TCP half-open scan by fully completing the TCP connection.
TCP connect scans require lower user privileges to execute, making them more accessible to potential threat actors.
However, because this technique establishes full TCP connections, they are readily detectable by Intrusion Detection Systems (IDS). Sophisticated cybercriminals are therefore likely to avoid this more conspicuous reconnaissance method.
UDP Scan: Service Discovery
UDP scans are employed to identify active services on UDP ports.
For example, sending a DNS request packet to port 53 (a UDP port) can confirm if a DNS server is hosted on a target computer. A DNS reply validates the presence of a DNS server.
XMAS Scan: Evasive Technique
XMAS port scanning is favored by cybercriminals for its stealth, as it is often not logged in network activity and is less noticeable to firewalls.
After a TCP 3-way handshake, a client sends a FIN packet to signal the end of data transfer.
An XMAS scan sends packets with FIN, PSH, and URG flags set (hence “XMAS tree” due to the flags being “lit up”). No response to an XMAS scan typically indicates an open or filtered port. An RST (Reset) response indicates a closed port.
Cybercriminal Reconnaissance: Weaponizing Port Scanning
Port scanning is a cornerstone tool for cyber attackers during reconnaissance phases.
Cybercriminals leverage port scanning to evaluate the security posture of a prospective victim before launching an attack.
Key information sought includes:
- Firewall presence and configuration.
- Security vulnerabilities within the target network.
- Exploitation difficulty for identified vulnerabilities.
- Port status: open, closed, or filtered.
Port scanning responses reveal how different parts of your network interact with a hacker’s probes. This information is then used to assess the overall cybersecurity strength of the targeted network areas.
Detecting malicious port scanning is challenging. Advanced cybercriminals can employ TCP protocol techniques to camouflage their activities within legitimate-looking decoy traffic.
Top 5 Free Open Port Checking Tools
Below is a curated list of 5 best free open port scanning tools. Each tool description includes a link to a YouTube tutorial for practical guidance.
1. Nmap: The Network Mapper
Nmap (Network Mapper) stands out as a highly acclaimed, free, and open-source port scanning tool. It provides a wide array of port scanning techniques, including TCP half-open scans, making it a versatile choice for comprehensive network analysis.
Key Features:
- Diverse port scanning methodologies.
- Comprehensive open port identification on target servers.
- Operating system detection capabilities.
- Network service discovery.
- Remote port scanning functionality.
- TCP SYN Scanning for stealthier scans.
- UDP and TCP port scanning support.
- Scalability for scanning extensive networks with tens of thousands of devices.
- Cross-platform compatibility: Windows, Linux, macOS, Solaris, OpenBSD, FreeBSD, and more.
Download Nmap
Download Nmap for free here.
Nmap Tutorial
Learn how to effectively scan for open ports using Nmap with this video tutorial: Nmap Tutorial.
2. Wireshark: The Network Sniffing Powerhouse
Wireshark is a leading free network sniffing tool, primarily used to detect malicious network activity and analyze traffic. It can also be effectively utilized to identify open ports by examining network communications.
Key Features:
- Detailed visibility into request and reply streams for each port.
- Malicious network activity detection.
- Troubleshooting capabilities for high bandwidth consumption issues.
- Extensive data packet filtering options for focused analysis.
- Ability to follow and monitor specific data streams of interest.
Download Wireshark
Download Wireshark for free here.
Wireshark Tutorial
Discover how to leverage Wireshark for open port scanning with this video tutorial: [Wireshark Tutorial].
3. Angry IP Scanner: The Fast and Lightweight Option
Angry IP Scanner is a free, open-source network scanner renowned for its speed and simplicity. It offers a suite of network monitoring functionalities, including efficient port scanning.
Key Features:
- Portable application – no installation required.
- Hostname resolution for easy network identification.
- Command-line interface for scripting and automation.
- Cross-platform compatibility: Windows, macOS, and Linux.
- Flexible export options for scan results: CSV, TXT, XML, and IP-Port list formats.
Download Angry IP Scanner
Download Angry IP Scanner for free here.
Angry IP Scanner Tutorial
Learn how to identify open ports using Angry IP Scanner with this video tutorial: [Angry IP Scanner Tutorial].
4. NetCat: The Command-Line Swiss Army Knife
NetCat is a versatile command-line utility for network tasks, including port scanning. It uses the TCP/IP protocol to interact across network connections, making it a powerful tool for network diagnostics and exploration.
Key Features:
- IP address usage detection.
- Port scanning capabilities.
- Tunneling modes for both UDP and TCP protocols.
Download NetCat
Download NetCat for free here.
NetCat Tutorial
Discover how to utilize NetCat to identify open ports with this video tutorial: [NetCat Tutorial].
5. Advanced IP Scanner: The Windows-Centric Solution
Advanced IP Scanner is a free Windows-based tool designed for analyzing IP addresses and ports within local networks (LANs). It offers a user-friendly interface and efficient scanning capabilities.
Key Features:
- Resource availability checking on network devices.
- Optimized for LAN analysis.
- Automatic detection of MAC addresses.
- Exportable scan results in CSV format.
- Access to shared resources on discovered devices via HTTP, HTTPS, FTP, or shared folders.
- Portable execution option available.
Download Advanced IP Scanner
Download Advanced IP Scanner for free here.
Advanced IP Scanner Tutorial
Learn how to scan for open ports using Advanced IP Scanner with this video tutorial: [Advanced IP Scanner Tutorial].
UpGuard: Elevating Your Port Scanning and Security Strategy
While port scanners are valuable for identifying potential security weaknesses, they represent a reactive, legacy approach to security. For proactive and reliable exposure detection, an attack surface management (ASM) solution like UpGuard is highly recommended.
UpGuard provides continuous scanning of both internal and external threat landscapes to proactively discover and remediate vulnerabilities before they can be exploited by cybercriminals. UpGuard’s capabilities extend to detecting and mitigating data leaks associated with third-party vendors, significantly reducing the risk of supply chain data breaches. By implementing a robust ASM solution, you move beyond basic port scanning to achieve comprehensive and proactive cybersecurity.
