Enter any domain name, or host ip address you would like to perform a 'Portscan' on (i.e. isc.org or 220.127.116.11)
A Port Scanner is a piece of software (in w3dt's case a cgi-driven webpage) designed to search a network host for open ports. This is often used by administrators to check the security of their networks and by hackers to compromise it.
W3dt's Web Based Port Scaner uses SYN TCP Scanning. An SYN scan is the most popular form of TCP scanning. Rather than use the operating system's network functions, w3dt's port scanner generates raw IP packets itself, and monitors for responses. This scan type is also known as "half-open scanning", because it never actually opens a full TCP connection. The port scanner generates a SYN packet. If the target port is open, it will respond with a SYN-ACK packet. The scanner host responds with a RST packet, closing the connection before the handshake is completed.
Many Internet service providers deny their customers the ability to perform port scans outside of their home networks. This is usually covered in the Terms of Service or Acceptable Use Policy to which the customer must have already agreed. Other public and private networks may also place such limitations upon their users.
Some ISPs implement Packet filters or Transparent proxies that prevent outgoing port scans being able to access certain ports. For example, if an ISP provides a transparent HTTP proxy on port 80, port scans of any address will appear to have port 80 open, regardless of target host's actual state.
The w3dt Portscan tool is based off code from the open source nmap tool.