How to Scan Open Ports on a Host with Python

A Python function that uses socket.connect_ex to check for open TCP ports on a given host within a range and returns a list of open ports.

Medium Python 3.9+ Jun 27, 2026 Automation & scripting 1 views 0 copies

socket network port-scanning security tcp automation

Python code

22 lines

Python 3.9+

import socket

def scan_ports(host, start_port, end_port):
    open_ports = []
    for port in range(start_port, end_port + 1):
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.settimeout(0.5)
        result = sock.connect_ex((host, port))
        if result == 0:
            open_ports.append(port)
        sock.close()
    return open_ports

if __name__ == "__main__":
    target_host = "localhost"
    start_port = 1
    end_port = 1024
    ports = scan_ports(target_host, start_port, end_port)
    if ports:
        print(f"Open ports on {target_host}: {ports}")
    else:
        print(f"No open ports found on {target_host} in range {start_port}-{end_port}.")

Output

stdout

Open ports on localhost: [80, 443]

How it works

The socket.connect_ex method returns an error indicator — 0 means the connection succeeded, indicating the port is open. A short timeout (0.5 seconds) keeps the scan fast. The function iterates over every port in the range, creating a new socket each time, which is simple but not optimized for speed. Using AF_INET and SOCK_STREAM specifies an IPv4 TCP connection. After each check the socket is closed to free system resources.

Common mistakes

Forgetting to set a timeout, which can cause the script to hang on unresponsive ports
Not closing the socket after each attempt, leading to resource leaks
Assuming a refused connection always means the port is closed (some services reject without SYN-ACK)
Using a too-large port range without any parallelism, making the scan very slow

Variations

Use `socket.setdefaulttimeout(0.5)` globally instead of per-socket timeout
Implement threading or asyncio to scan multiple ports in parallel for faster results

Real-world use cases

Security audits: quick internal network scans to discover unexpected open ports.
DevOps health checks: verify that expected services (e.g., SSH, HTTP) are reachable from a monitoring box.
Pentesting scripts: map attack surface before manual testing in a controlled environment.

How to Scan Open Ports on a Host with Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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