check open ports,check for listening ports tool,domain/hostname/IP/URL

On this page you can check opened ports of domain/hostname/IP, it's the same as check listening of ports.
This tool trying to create port connection, connection timeout is 4 sec.
How to check available ports: just input destination in first field, and port (in listbox) or list of ports (in third field). After submit tools will scan ports.
Port Scanning is one of the most popular service of attackers, they use it to discover services they can break into. All machines connected to a Local Area Network (LAN) or Internet run many services that listen at well-known and not so well known ports. Essentially, a port scan consists of sending a message to each port, one at a time. The kind of response received indicates whether the port is used and can therefore be probed further for weakness.
A port scanner is a software application designed to probe a network host for open ports. To portscan a host is to scan for listening ports on a single target host. *This tool check ip open port too. You can scan port from list that is near fisrt input field too, but if third field is inputed - tools will use it, but not value from listbox.
Sometime if need to know some web server is dead or not, this tool can help you, because if www-80 is closed - it is when server is www inaccessible, else it's stay to recieve connections.

WIKI links :

what is port scaner, open port checker, how it's working