What is the difference between TCP and UDP ports?

TCP (Transmission Control Protocol) establishes a confirmed connection before sending data, making it reliable but slightly slower — used by HTTP, HTTPS, SSH, and most application protocols. UDP (User Datagram Protocol) sends data without a handshake, making it faster but less reliable — used by DNS, streaming, and gaming.

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Type an IP address or domain name into the Target Address field, pick a scan mode — single port, port range, or a preset like Web Services — and hit Scan. You'll get back the port status (open, closed, or filtered), the detected service running on it, and the response time in milliseconds. Need to check your own machine? Click Use My IP and your public IP fills in automatically. Need to test many hosts at once? Try our bulk port scanner for multiple IP addresses.

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What Is a Port in Networking?

A port is a virtual communication endpoint that lets your operating system route incoming and outgoing network traffic to the correct application. Every program that sends or receives data over a network claims at least one port. Your browser, email client, SSH session, and database all use separate port numbers so they can share the same IP address without interfering with each other.

Ports are identified by a number from 0 to 65535. When you type https://example.com in your browser, your OS opens a connection to port 443 on the remote server — that's the standard HTTPS port. The server sees the incoming connection on 443 and knows to hand it to the web server process.

TCP vs UDP Ports

Every port number exists in two separate namespaces — TCP and UDP — so port 53 TCP and port 53 UDP are technically different endpoints, though DNS uses both.

TCP — Transmission Control Protocol

Establishes a confirmed three-way handshake before any data flows. Reliable and ordered — every packet is acknowledged. Used by HTTP, HTTPS, SSH, FTP, SMTP, and most application protocols.

UDP — User Datagram Protocol

Sends data without a handshake — no confirmation, no guaranteed delivery. Faster with lower overhead. Used by DNS, VoIP, video streaming, online gaming, and NTP.

Port Number Ranges

Range	Name	Examples
0 – 1023	Well-known ports	HTTP (80), HTTPS (443), SSH (22), FTP (21)
1024 – 49151	Registered ports	MySQL (3306), PostgreSQL (5432), RDP (3389)
49152 – 65535	Dynamic / ephemeral	Temporary client-side ports assigned by the OS

What Is a Port Checker and How Does It Work?

A port checker (also called a port scanner or open port checker) is a tool that probes a specific IP address or domain on one or more port numbers and reports whether each port is open, closed, or filtered. This tool runs checks from our servers, so what you see reflects how the internet reaches your host — not how your local machine sees it.

How Does an Online Port Scanner Work?

Most online port checkers use a TCP SYN probe (sometimes called a half-open scan):

The scanner sends a TCP SYN packet to the target IP and port.
If the port is open, the target responds with SYN-ACK. The scanner records "open" and sends RST to close the half-open connection.
If the port is closed, the target responds with RST immediately.
If the port is filtered, a firewall drops the packet silently — the scanner times out with no response.

Response time in milliseconds is measured from when the probe is sent to when the first packet is received.

How to Find Your Port Number (Without a Scanner)

To see which ports are already open on your own machine, use the built-in network tools:

Windows

netstat -ano

Lists active connections, listening ports, and the PID of each process. Open Task Manager to match PIDs to application names.

Linux / macOS

ss -tuln

Shows all TCP and UDP listening ports with no DNS resolution. Add -p to also print process names.

Most Common Ports to Check

Not all ports carry the same risk or importance. The ones below are the most frequently scanned because they correspond to services that are either universally deployed or commonly exploited.

Essential Service Ports

Port	Protocol	Service	Why It Matters
21	TCP	FTP	File transfer — transmits credentials in plain text; often targeted
22	TCP	SSH	Secure remote shell access; high-value brute-force target
23	TCP	Telnet	Legacy remote access — unencrypted; should always be closed
25	TCP	SMTP	Email delivery; open relay misconfiguration enables spam abuse
53	TCP/UDP	DNS	Domain resolution; open resolvers can be used in amplification attacks
80	TCP	HTTP	Plain-text web traffic; should redirect to HTTPS (443)
443	TCP	HTTPS	Encrypted web traffic; must be open for any public web server
3389	TCP	RDP	Windows Remote Desktop — extremely targeted; restrict to VPN only
5900	TCP	VNC	Remote desktop sharing — exposing to the internet is a critical risk
8080	TCP	HTTP Alt	Common dev/proxy port; often forgotten and left exposed

Database and Application Ports

Port	Service	Risk if Exposed
1433	Microsoft SQL Server	Direct DB access; frequent ransomware target
3306	MySQL / MariaDB	Exposed DBs are routinely scanned and dumped
5432	PostgreSQL	Should be firewalled; never exposed to the internet
6379	Redis	No auth by default; exposed Redis instances lose all data
27017	MongoDB	Misconfigured open installs led to mass data breaches
9200	Elasticsearch	No auth by default; exposes full index to anyone

Who Uses a Port Checker?

Port scanning isn't just a security professional's tool. Anyone who sets up a server, troubleshoots a network connection, or configures a home router will reach for a port checker at some point.

Developers and IT Teams

Verify that a newly deployed API, microservice, or server is reachable on the correct port before releasing to production. Confirm that TLS termination is happening on 443 and not leaking traffic on 80. Debug "connection refused" errors by checking whether the port is open at all versus blocked by a firewall.

Network and Security Professionals

Run quick external port scans as part of vulnerability assessments to confirm firewall rules are enforced. Validate that database ports (3306, 5432, 27017) are not exposed to the internet. Check that legacy services like Telnet (23) or FTP (21) are closed on public-facing infrastructure.

System Administrators

Audit open ports on servers after applying firewall rule changes. Confirm that RDP (3389) or SSH (22) are firewalled or limited to specific IPs. Verify that VPN, monitoring, and backup services are listening on their expected ports after a system update or reboot.

Home Users and Gamers

Test whether port forwarding is working correctly for a game server, home NAS, or self-hosted application. Confirm that the router's port mapping is set up right and that the ISP isn't blocking specific ports. Diagnose why friends can't connect to a Minecraft, Valheim, or Plex server.

What Is Port Forwarding and How Do You Test It?

Port forwarding (also called port mapping) instructs your router to pass all traffic arriving on a specific external port to a particular device inside your private network. Without it, your router's NAT keeps every incoming connection blocked by default — your internal servers are invisible from the internet.

Common use cases include hosting a game server, running a home web server, accessing a NAS remotely, or connecting to a home machine via SSH or RDP. Your ISP-assigned public IP receives the connection, and the router forwards it to the private IP of your target device.

How to Verify Port Forwarding Is Working

Set up the port forward rule in your router's admin panel — specify the external port, internal IP, and internal port for the target device.
Start the service on the target device (game server, HTTP server, SSH daemon, etc.) and confirm it's listening locally using ss -tuln or netstat -ano.
Check the local firewall on the target device — Windows Defender Firewall or ufw/iptables on Linux may be blocking the port even if the router is forwarding correctly.
Run this port checker using your public IP address and the forwarded port number. An "open" result confirms the full path is working — router, firewall, and service.

Note: some ISPs block specific ports (25, 80, 443) on residential connections. If the port shows as filtered and you've confirmed the router and firewall settings are correct, contact your ISP or consider switching to a non-standard port.

Frequently Asked Questions

What is a port in networking?

A port is a virtual endpoint that lets your operating system direct network traffic to the correct application. Ports are numbered 0–65535. Well-known ports (0–1023) are reserved for common services — HTTP uses 80, HTTPS uses 443, SSH uses 22. When you connect to a website, your OS opens a connection to the server on the appropriate port; the server routes that traffic to the web server process.

What is the difference between TCP and UDP?

TCP (Transmission Control Protocol) establishes a confirmed connection before sending data — every packet is acknowledged and re-sent if dropped. This makes it reliable but slightly slower. HTTP, HTTPS, SSH, and most application protocols use TCP. UDP (User Datagram Protocol) sends packets without a handshake or acknowledgement — faster and lower overhead, but no guaranteed delivery. DNS, video streaming, VoIP, and online gaming use UDP.

How many ports does a computer have?

Each computer has 65,536 TCP ports and 65,536 UDP ports — 131,072 total. In practice, only a small number are in active use at any time. Most well-known services live in the 0–1023 range, with application and database services typically using registered ports (1024–49151).

What does "filtered" mean when checking a port?

A filtered port means a firewall or security device is silently dropping connection packets — no response comes back and the scanner times out. This is different from closed (the host actively responds with a rejection packet) or open (the host accepts the connection). Filtered is harder to distinguish from a downed host than closed is.

What is port forwarding and how do I test it?

Port forwarding tells your router to pass incoming traffic on a specific port to a device inside your private network. To test it: set up the port forward rule in your router admin panel, start the service on the target device, confirm the local firewall isn't blocking the port, then use this tool to scan your public IP on that port. An "open" result means the full path is working.

Are open ports a security risk?

Open ports are only a risk if the service behind them is unpatched, misconfigured, or unnecessary. Every open port is a potential entry point — attackers scan for open ports constantly as a first reconnaissance step. The rule of thumb: keep software updated, restrict access using firewall rules, and close any port whose service you don't actively need.

Is port 80 safe to leave open?

Port 80 is safe to leave open if you're intentionally running an HTTP server — for example, to redirect visitors to HTTPS. The risk comes from running an outdated or misconfigured web server, not from the port number itself. If you don't operate a web server, port 80 should be closed. Never serve actual content over plain HTTP — always redirect to 443.

How do I find open ports on my own computer?

On Windows, open Command Prompt and run netstat -ano — this lists all active connections and listening ports alongside the PID of the owning process. On Linux or macOS, run ss -tuln to see all TCP and UDP listening ports. Add -p to also display process names.

How do I close an open port?

First, stop or disable the service that is listening on the port. Then configure your firewall to block that port number — Windows Defender Firewall, ufw on Ubuntu, or iptables on other Linux systems. On cloud servers, also check your provider's security group or network ACL rules — traffic can be blocked at the infrastructure level independently of the OS firewall.

Can I use this to check someone else's server?

You can scan public-facing IPs and domains you own or have explicit written permission to test. Scanning servers you don't own without authorization may violate the Computer Fraud and Abuse Act (US), the Computer Misuse Act (UK), or equivalent laws in your jurisdiction. This tool is designed for legitimate network diagnostics — always get permission before scanning third-party infrastructure.