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An online port scanner is a web-based application that will monitor a specified computer, server or any other host for open ports. It is most often used for security purposes, to check for vulnerabilities on a router, firewall, server or computer. Online port scanners are often free and available to use publicly.
A port, with regard to a computer, is an entryway for computer traffic to access a network or computer. It is an access point for a protocol, which is basically the language used in networking. The protocol, called the Internet Protocol Suite, has several layers to it; the layer used by ports is the transport layer. Ports are also assigned numbers to identify them, so applications and processes know where to go to access the computer or network.
Many ports are commonly used by specific services or applications to route their specific traffic over the Internet or internal network. For instance, hypertext transfer protocol (HTTP), the protocol used in the World Wide Web, is typically associated with port 80. Web servers will listen on that port for incoming traffic, and most browsers are set to use port 80 by default.
When a port is scanned, it is being checked to see if it is open, closed or blocked. If a port is open, it can be exploited, or connected to, by an application or process. Port scans are generally done by either hackers, people looking to maliciously exploit the port, or by someone trying to protect the port, such as a network administrator. If a network administrator detects a port is open that shouldn’t be, they can simply program the computer or router to block, or close, that port. They can also program it to filter only certain types of traffic through it.
An online port scanner is typically hosted by a company that offers port scanning capabilities over the Internet. This is most helpful with smaller networks, such as a home network, as they are often limited in scope, and it is not worth it financially or practically to purchase port scanning software. The online port scanner will usually do a port scan looking for open, closed and blocked ports. Some of them can also list what ports are in use and by whom, according to the Internet Protocol (IP) address listed. This can help identify any vulnerabilities or if someone is accessing the network or computer.
Many Internet service providers (ISPs) block port scanning on larger scales. This prevents one user from accessing another user in the ISP’s network using a port scan or online port scan. Typically, most ISPs will allow an online port scanner to scan the originating requestor’s network only.