Blog
Guides, tips, and tutorials for Mac port monitoring and network diagnostics.
How to Check If a Port Is Open on Mac
"Is this port open" is two questions: listening locally, or reachable from another host. Here's how to test both on Mac with lsof, nc, and nmap.
'Address Already in Use' on Mac: Fix EADDRINUSE and Free the Port
Address already in use (EADDRINUSE) means another process holds the port. Here's how to find it on macOS, free the port, and stop it recurring.
What Is Port 8080 Used For?
Port 8080 is the most common alternative to port 80 for HTTP, used by proxies, dev servers, and Tomcat. Here's what it does, why it exists, and how to see what's using it on your Mac.
netstat on Mac: The Complete Guide
How to use netstat on macOS to see ports, connections, routing, and network stats, plus why macOS netstat can't show process names and what to use instead.
Port Number Ranges: Well-Known, Registered & Ephemeral
Port numbers run 0 to 65535, split into three ranges by IANA. Here's what well-known, registered, and dynamic/ephemeral ports are, where the boundaries fall, and why it matters.
What Is localhost? 127.0.0.1 Explained
localhost is your own computer talking to itself. Here's what 127.0.0.1 means, why localhost:3000 works, the difference between localhost and your LAN IP, and common gotchas on Mac.
What Is a Port? Networking Explained Simply
A network port is a numbered doorway that lets one computer run many network services at once. Here's what ports are, why they exist, and how the numbering works, with real examples.
TCP vs UDP: What's the Difference?
TCP is reliable and ordered; UDP is fast and connectionless. Here's how the two transport protocols actually differ, when each is used, and how to tell which one a port is running.
Port Forwarding on Mac: Router, Firewall, and How to Verify It Worked
Port forwarding on Mac requires two separate steps: configuring your router and configuring macOS. Here's how to get both right and confirm the port is reachable.
Mac Network Monitor: 4 Tools for 4 Different Questions
Activity Monitor, Portie, Little Snitch, and Wireshark each answer a different network question on Mac. Here's how to pick the right one for what you're trying to do.
How to List All Open Ports on Mac
Three ways to list every open port on your Mac: lsof, netstat, and a GUI. Includes commands for TCP, UDP, and filtering by process or port number.
Port 3000 Already in Use on Mac: How to Find and Free It
Getting EADDRINUSE on port 3000? Here's how to find what's holding the port and kill it, with or without Terminal.
How to See Which App Is Using a Port on Mac
macOS won't tell you which app owns a busy port. Here's how to find out with lsof and a dedicated GUI tool.
Port 5000 in Use on Mac? It Is AirPlay Receiver. Here Is the Fix
On macOS Monterey and later, Control Center holds port 5000 for AirPlay Receiver, blocking Flask and other dev servers. Turn it off or move your server.
Kill a Process on a Port on Mac: Find the PID and Free It
Find which process is using a port on your Mac and kill it with lsof and kill -9, including the macOS services that just relaunch themselves.
macOS Network Utility's Port Scanner Is Gone. Here Are Your Options.
Apple removed Network Utility in macOS Big Sur. Here's what to use instead for scanning ports on a Mac, with and without the Terminal.
Common Ports and What They're Used For
A practical reference for common TCP and UDP port numbers: web, SSH, email, databases, dev servers, and macOS-specific ports.
What Are Ephemeral Ports? Why Your Mac Has So Many Open
Open a port monitor on macOS and you'll see dozens of ports in the 49152-65535 range. Here's what they are, why they exist, and whether to worry.