Check whether a single TCP port is reachable on a host — a quick connectivity test for firewalls, security groups, and services during an incident.
Common
🎓Learn: TCP port checks
✓ open
A service is listening and completed the TCP handshake.
✗ closed
The host answered but refused the connection (RST) — nothing is listening.
⏳ filtered
No answer at all — a firewall/security group is silently dropping packets.
Open vs closed vs filtered — why it matters▾
“Closed” means you reached the host but the port has no listener — often a stopped service. “Filtered” means a firewall or security group is dropping the traffic before it ever reaches the service, so you get silence instead of a refusal.
This is a single-port TCP connect test▾
It opens one TCP connection to the port you choose and reports whether the handshake succeeds — the same thing nc -vz host port does. Private and reserved addresses are refused for safety.