What is the netstat Command?

Quick Answer: The netstat command shows active network connections, ports that are listening, routing tables, and network interface stats on your system. It's a built-in diagnostic tool that helps you monitor network activity and troubleshoot connection issues.

Network monitoring lets you spot issues and keep your systems running smoothly. Without visibility into what's happening on your network connections, you're basically flying blind when problems pop up. The netstat command gives you real-time visibility into what is actually happening.

On its surface, netstat looks like a simple network command that displays information on the command line. But the information that it displays is very detailed and very helpful. It shows you which connections are active, what ports are listening, and how data flows through your system. Every network admin needs this tool in their toolkit because it shows you what your network is actually doing right now.

The netstat command has been around for a very long time on Windows, Linux, and Unix systems, so it's nothing new. But it's reliable, fast, and doesn't need any installation—it comes with your OS right out of the box. You can start using it right away to examine your network connections.

What is the netstat Command?

The netstat command is short for "network statistics." It runs from the command line and shows detailed information about your system's network connections and activity in real time. When you run netstat, you get a snapshot of what's happening on your network at that exact moment, which makes it a great troubleshooting tool.

Netstat reads information from your operating system's network stack. Your OS has to keep track of every connection and open network port, along with all of the system's network interface data. The netstat command formats this information and shows it in an easy-to-read format.

One thing that might surprise you about netstat is just how simple it is. One command gives you access to connection details that you would usually need a commercial tool to analyze. Granted, you might not have advanced features that can directly interact with this data, but seeing it is very useful. You can also see which applications are using the network, what remote systems you're connected to, and if your services are listening on the right ports.

Network admins use netstat for daily troubleshooting tasks before they need to use the fancier tools. If users complain about slow connections or suspicious network activity that needs investigating, netstat is normally the tool that gives us the first clues about what's really going on.

How Does netstat Work?

Netstat gathers information from different sources inside your operating system's network subsystem. It has access to connection tables, routing information, and interface stats that your system maintains on its own. Your OS needs to track all of this information as it navigates file shares, internet browsing, and any other service that needs access to the network.

The netstat command has three main functions that cover most basic network monitoring:

Displaying Active Connections 

When you run netstat, it shows every active network connection on your system. Connections are always in different states of setup or teardown, and then there are connections waiting for data. You'll see which local ports are connected to which remote addresses and hopefully spot suspicious connections at a glance.

Showing Routing Tables 

Your system keeps up-to-date routing tables, which decide where network traffic should go. Netstat can show these tables so you can verify that traffic is taking the right path through your network. This helps when troubleshooting connectivity issues or checking that your network configurations are working properly.

Monitoring Interface Statistics 

Network interfaces collect statistics about all the data they handle. With the right command switches, netstat can show packet counts and error rates, along with other performance metrics, for each interface on your system. The output helps you find performance bottlenecks or hardware issues.

Understanding netstat Output Fields

The standard netstat output has a few columns that tell you different things about each connection:

• Proto (Protocol): This shows whether the connection uses TCP or UDP. TCP establishes a connection and confirms that the data has been delivered, whereas UDP just blasts packets out without waiting for confirmation.

• Local Address: Displays your system's IP address and port number for this connection. The format is typically IP:port, like 192.168.1.100:80.

• Foreign Address: This shows the remote system's IP address and port that you're connected to. It tells you exactly where your data is going or coming from.

• State: This tells us about the current status of TCP connections. The most common states are: 

• ESTABLISHED (active connection)

• LISTENING (waiting for incoming connections)

• TIME_WAIT (connection closing down)

Reading a netstat Example

Here's what a netstat line looks like:

TCP    192.168.1.100:443    203.0.113.45:52847    ESTABLISHED

This shows a TCP connection from your local system (192.168.1.100) on port 443 (HTTPS) to a remote system (203.0.113.45) on port 52847. The connection is currently established, which means it is probably also actively transferring data.

What is netstat Used for?

The short answer is that network pros use netstat for important tasks that keep systems running smoothly and securely, and it has other uses too:

Identifying Active Connections and Open Ports

Netstat is often used to check what connections are currently active on a system. When you need to verify that a service is running and accepting connections, netstat shows you right from the command line. You can quickly spot which ports are listening and which ones have active connections.

When troubleshooting connectivity issues, netstat can provide important information right away. For example, if users can't reach a web server, running netstat on that server quickly shows whether it's actually listening on port 80 or 443. 

Monitoring Network Performance and Traffic

Netstat shows us interface statistics that tell a story about how much traffic your network interfaces are handling. You can see packet counts, error rates, and dropped packets. When your network feels slow, netstat statistics can help you get enough information to pinpoint the bottleneck. High error rates could indicate hardware problems, or excessive dropped packets could indicate bandwidth issues.

Detecting Security Issues

Security experts use netstat to spot suspicious network activity, such as unexpected connections to unknown remote systems. These could indicate malware or unauthorized access attempts. 

Regularly checking netstat output lets you establish a baseline of normal activity. When things are above that level, then you will quickly spot the difference and start investigating further. Connections to suspicious IP addresses or unusual port usage shouldn’t be ignored on your network, and netstat gives you the first glimpse that something needs to be investigated. 

What are the Advanced Options and Parameters for netstat?

The basic netstat command is helpful, but there is more to unlock with different parameter switches. Adding parameters gives us a much more detailed view and specific displays of your network activity.

Essential netstat Parameters

Netstat -a shows all connections and listening ports. This is probably the most useful parameter because it gives you a complete picture of the network’s activity. Without -a, you only see established connections and you miss seeing the listening services.

Netstat -n displays IP addresses instead of resolving hosts and ports to names. This makes the output faster and avoids DNS lookups that might fail if you are having network issues, and they can take time. When you need quick results and you just want to see IP addresses, then -n is a good option.

Netstat -r displays the routing table. This table shows how your system decides where to send network traffic. If you're having connectivity issues with specific networks, checking the routing table can hint at where the problem is.

Combining Parameters for Better Results

You can combine different parameters to get exactly what you need:

• Netstat -an gives you all connections with IP addresses: This combination works for monitoring scenarios where you need to see everything without waiting for name resolution.

• Netstat -rn shows the routing table with IP addresses: This helps when you need to quickly look at routing information without DNS information.

• Netstat -e displays Ethernet stats on Windows systems: You'll see bytes sent and received, along with error counts for each network interface.

Platform-Specific Options

Different operating systems offer additional netstat options that have extra details. An important thing to note is that netstat is largely deprecated on modern Linux systems. It has mostly been replaced with ss and ip from the iproute2 package. You can still use netstat on Linux, but it may not be installed by default. You’ll need to install the net-tools package in most cases if you want to use netstat on modern Linux systems

On Windows, netstat -o shows the process ID (PID) for each connection. This lets you see exactly which app is using each network connection. 

On Linux systems, netstat -p (when run as root) shows process names and PIDs. This gives you the same data about which processes are using the network.

What are Common Issues and Limitations with netstat?

Netstat has been around for a very long time, but it has its limits of what it can and can’t show you. Here are some of the limitations that you should know about.

Information Overload

On busy systems, netstat output can be confusing most of the time, and sometimes overwhelming. A server might have hundreds of active connections, making it difficult to spot the information you actually need. 

This is where those parameters we mentioned earlier become valuable. They help you filter the output and get the data you want without filling your screen with information you don’t need to know.

When you're looking for specific connections, combine netstat with other command-line tools. On Unix-like systems, you can pipe netstat output through grep to find exactly what you're looking for.

Interpreting Complex Network States

TCP connections go through different states, and they can be confusing if you don't understand the TCP protocol. Seeing many connections in the TIME_WAIT state might look a bit abnormal, but it can be normal behavior as connections perform their clean-up. If you see CLOSE_WAIT, then you are looking at possible application errors where the app has closed, but the connection hasn’t been properly closed.

Connection state analysis takes practice, but there are a few general rules of thumb that you can use to get yourself started. ESTABLISHED connections are usually actively transferring data, LISTENING ports are waiting for incoming connections, and TIME_WAIT connections are finishing their cleanup process.

Limitations in Modern Environments

Modern containerized and virtualized environments can make netstat output more complicated to understand. This is because container networking creates additional network namespaces that might not show up in the host system's netstat output.

Cloud environments and software-defined networking add even more layers that traditional netstat can't see. If you are troubleshooting in a software-defined network (SDN), you might need more modern tools or some platform-specific commands to get the complete picture. In containerized environments like Docker, running netstat inside a container only shows that container's network connections, not the host system's full network picture.

When netstat Falls Short

Some network troubleshooting requires more advanced tools written with the limitations of legacy apps like netstat in mind. Packet capture tools like tcpdump or Wireshark can capture actual network traffic in as much detail as you want, and you can save these captures for even further investigation later. Network monitoring solutions provide you with historical data and alert you that netstat wasn’t designed for display.

For advanced bandwidth monitoring, tools like iftop or nethogs give you real-time traffic analysis that eclipses netstat's basic stats output. The CompTIA Network+ certification covers these advanced networking tools and when you should use each one.

Mastering netstat for Network Success

The netstat command gives you quick access to your network connections and activity. It's fast, reliable, and available on virtually every system. It's great for basic connection monitoring or initial security investigations and serves as the foundation for early network troubleshooting.

If you want to try it out for yourself, then start with basic commands like netstat -a to get a feel for the output format. Experiment with different parameters to see how they change the information on your screen, and try to think of what apps and systems are running on your network as you go through the output. The tool is easy to practice with, so you should definitely set aside some time to get familiar with it.

Network admins usually combine netstat with other tools when they want to build a full picture of a network and how healthy it is. It is great for quick checks and fast troubleshooting, but you’ll need specialized tools when you need to dig deeper.

Want to learn more about becoming a network professional? Consider the CompTIA Network+ certification online training to build comprehensive networking skills.