An ISDN line provides dial on demand Internet access from your site. This connection takes about 5 seconds. During the time the line is up you pay for a local rate telephone call. The lines may be numbered individually or they can both have the same telephone number. It determines the next hop to which the packet of data should be forwarded and maintains a table of routes and their conditions allowing it to decided which is the best path for the packet of data to take.
Many routers depending on their complexity allow the user to set up packet filtering.
For example it is usually normal to allow all outgoing TCP connections, all incoming SMTP and DNS to the mailhost and all FTP connections to high TCP ports usually greater than 1024. Security You need some form of security to protect your site from the millions of Internet users worldwide. By connecting your computer or corporate network to the Internet you are effectively giving anyone connected to the Internet direct access to your machines. One method of attack is for the hacker to use the connectivity and anonymity of the Internet to attack the operating system on your machine. Such attacks can either lead to the hacker damaging your machine or gaining access to the data stored on it.