Blocking content coming into and going out to the Internet. Accomplished by firewall software residing in a stand-alone device, router or user's machine or in a filtering program in each user's machine, content can be blocked based on website address or Internet service. For example, users might be allowed access to the Web but not to transfer files (see FTP).Websites and Internet ServicesIncoming content can be blocked for specific users or for all users. Blocked websites can be recognized by their Web address (URL) or by elements within the URL. All Internet traffic is identified by port number, and an entire service can be blocked by this number. For example, websites use port 80 and file transfers are 20, 21 and 22 (see well-known port).Outgoing requests to the Web can be compared with local and cloud-based databases of known malicious URLs. In addition, depending on company policy, lists of restricted website categories can be maintained such as shopping, news, streaming videos and pornography.In the home, parents can restrict their children's access with special browsers and filtering programs (see parental control software).