What is Web crawler?

A web crawler (also known as a web spider or web robot) is a program or automated script which browses the World Wide Web in a methodical, automated manner.

This process is called Web crawling or spidering.

Many legitimate sites, in particular search engines, use spidering as a means of providing up-to-date data.

Web crawlers are mainly used to create a copy of all the visited pages for later processing by a search engine, that will index the downloaded pages to provide fast searches.

Crawlers can also be used for automating maintenance tasks on a Web site, such as checking links or validating HTML code.

Also, crawlers can be used to gather specific types of information from Web pages, such as harvesting e-mail addresses (usually for spam).
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What is Port (computer networking)

In the internet protocol suite, a port is an endpoint of communication in an operating system. While the term is also used for female connectors on hardware devices (see computer port), in software it is a logical construct that identifies a specific process or a type of network service. A port is always associated with an IP address of a host and the protocol type of the communication, and thus completes the destination or origination network address of a communication session. A port is identified for each address and protocol by a 16-bit number, commonly known as the port number. For example, an address may be 'protocol: TCP, IP address: 1.2.3.4, port number: 80', which may be written 1.2.3.4:80 when the protocol is known from context. Specific port numbers are often used to identify specific services. _________________________

What is a localhost ?

In computer networking, localhost is a hostname that means this computer. It is used to access the network services that are running on the host via its loopback network interface. Using the loopback interface bypasses any local network interface hardware. The local loopback mechanism is useful for testing software during development, independently of any networking configurations. For example, if a computer has been configured to provide a website, directing a locally running web browser to http://localhost may display its home page. On most computer systems, localhost resolves to the IP address 127.0.0.1, which is the most commonly used IPv4 loopback address, and to the IPv6 loopback address. _________________________

What is Back Door?

A back door is generally a piece of code intentionally left by the developer of the software or firmware that allows access without going through the normal security process. Back doors may also be the result of different malware/virus attacks that leave a method for remote, unsecured access into a device once the malicious code has been executed. _________________________

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