--- title: What is a Network Port? (and how to monitor one) description: An IP address gets you to the right machine; a port gets you to the right service on it. Here's what ports are, the common ones, and why monitoring them catches outages a website check misses. canonical: https://watchfor.io/blog/what-is-a-network-port --- [All posts](/blog) [Networking](/blog/category/networking) Feb 14, 2026 · 3 min read · WatchFor Team # What is a Network Port? (and how to monitor one) An IP address gets you to the right machine; a port gets you to the right service on it. Here's what ports are, the common ones, and why monitoring them catches outages a website check misses. You know that an [IP address](/blog/what-is-an-ip-address) points to a machine on the network. But a single machine runs many services at once — a web server, a database, SSH, mail. How does traffic reach the right one? Ports. If the IP address is a building's street address, the port is the room number. Let's make that concrete. ## What a port is A network port is a number (0–65535) that identifies a specific service on a machine. When you connect to example.com:443 , you're reaching the host at that IP, on port 443 — where its web server is listening for HTTPS. Every networked service listens on a port. The combination of IP + port is what actually pinpoints a service. ## The ports you'll meet A handful of "well-known" ports come up constantly: Port Service What it's for 80 HTTP Unencrypted web traffic 443 HTTPS Encrypted web traffic 22 SSH Secure remote login 25 / 587 SMTP Sending email 53 DNS Name resolution 3306 / 5432 MySQL / PostgreSQL Databases 6379 Redis Cache / key-value store When someone says "the database is on the default port," they mean one of these. ## Open, closed, or filtered A port is in one of three states, and the difference matters when debugging: State Meaning Open A service is listening and accepting connections Closed Nothing is listening — connection refused (instantly) Filtered A firewall is silently dropping traffic — connection hangs That distinction explains a lot: a [connection refused](/blog/connection-refused) is "closed" (instant no), while a hanging timeout is usually "filtered" (a firewall dropping packets). ## Why monitor a port directly? A website check tells you HTTP is working. But plenty of critical things aren't websites — a database, a mail server, a custom TCP service, a game server. Port monitoring opens a real connection to a host and port to confirm the service is actually accepting connections. This catches a class of problem a page check misses entirely: - A database that stopped accepting connections (even though the website is up). - A mail server whose SMTP port closed. - A service that crashed but the web tier in front of it is fine. A [TCP check](/blog/tcp-vs-udp) can even time the handshake, so you also catch a port that's open but slow . ## A quick how-to To monitor a port, you point a check at a host and port and confirm the connection succeeds: - Pick the host and port (e.g. your DB host on 5432 ). - Have the monitor open a TCP connection on a schedule. - Alert if it's refused, filtered (times out), or slow to connect. - Check from [multiple locations](/blog/global-monitoring-locations) so a single network hiccup doesn't page you. ## The bottom line In one line What A number identifying a specific service on a machine (IP = building, port = room). Common 80/443 web, 22 SSH, 25 mail, 53 DNS, DB ports. States Open (listening), closed (refused), filtered (dropped). Monitor it Confirm the service accepts connections — catches non-web outages. Ports are how one machine offers many services without them colliding. And because not everything important is a website, monitoring the right ports catches outages — a dead database, a closed mail port — that a homepage check would never see. Test a port instantly with the free [port checker](/port-checker), or set up continuous checks with [network monitoring](/docs/monitors/network). [#networking](/blog/tag/networking)[#basics](/blog/tag/basics)[#monitoring](/blog/tag/monitoring) ## Monitor your TCP ports Confirm services accept connections and respond fast, from multiple regions, around the clock. [Learn more](/tcp-monitoring)[Start free](/auth/sign-up) Share this article --- Canonical page: https://watchfor.io/blog/what-is-a-network-port · Site guide: https://watchfor.io/llms.txt