Check one port on another server
Checks whether one port on a server or domain of your choosing is open from the outside. There are four answers and they mean different things: open, closed (the connection was explicitly refused), no reply (the packet went unanswered, which usually means a firewall) and no route (this server's operating system had no path to the address). To stay accountable, every run is written to the server log with its target and port.
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What the one-port check measures
This tool opens one TCP connection to one port on a server or domain of your choosing and reports how the attempt ended. The input is host:port, such as example.com:443; an IPv6 address goes in brackets. The name is resolved, the address is checked against the reserved ranges, and the connection is made from the IPVIEW origin nearest you, inside Iran for visitors there and in Germany otherwise. The result says where it ran.
One port per request, never a range: a range is a scanner and a port is a question, and asking about a hundred ports costs a hundred requests against six a minute. Every run is written to the server log with its target, port and the caller's address, the one exception on this site to not recording who asked what, because a tool that reaches other people's servers has to be accountable.
How to read the result
Four verdicts. Open: something is listening and the connection was made. Closed: the server answered that nothing listens there, which proves the host itself is up. No reply: the packets went out and nothing came back within six seconds, which usually means a firewall dropping them, not a server that is off. No route: the operating system reported the network or host as unreachable, for example when this server has no route to an IPv6 address.
Below the verdict are the host, the address connected to, the port, the usual service for that port number, the response time and the raw network code such as ECONNREFUSED. The service name is a label from the number, not a detection: port 443 is called HTTPS whatever is listening. A closed port answers fast and a filtered one takes the whole timeout.
When it is useful
You have opened a port in a cloud firewall and the application still cannot be reached. Closed means the rule works and the service is not running; no reply means the rule is not in effect yet or a second firewall sits in front. The distinction saves restarting the wrong thing.
A mail server refused by some senders: check 25 and 587 from here, then Email blacklist for the address and Reverse DNS for its PTR name. If a port answers from Germany but not from Iran, the Reachability check on the domain says whether the name itself is being interfered with. For all the common ports on your own address, Port check does that.
Common questions
What is the difference between closed and no reply?
Closed is an answer: the far end sent a refusal, so the packet arrived and the host is up. No reply is silence within six seconds, which is what a firewall looks like.
Can I check a range of ports on another server?
No. One port per request, six requests a minute. The common-port scan on this site runs only against your own address.
Is the check logged?
Yes. The target, the port and your address are written to the server log before the connection is made, because this is the one tool here that connects to servers you do not own.
Why does it refuse a private address?
Reserved ranges such as 10.0.0.0/8 or ::1 are refused by every tool here that opens a socket. From this server those addresses are not yours; they are whatever sits next to the server doing the checking.
Limits. The DNS, WHOIS and SSL tools allow up to 20 requests a minute per IP address. Ping, traceroute and MTR send real packets to their target, so they allow 6 a minute.
The port check runs only against your own IP address. That is deliberate: the question people actually have, "is this port reachable from outside?", is answered by it, while this server cannot be used to scan anyone else's network.
Private and reserved addresses. These are refused by the tools that really connect to their target: the SSL check, ping, traceroute, MTR and the port check. The DNS tools have no such limit, because they only ask a nameserver a question and never connect to the address itself.