Ans : 

In both Netscape and Microsoft Internet Explorer version 3, the security manager is both very restrictive and not subject to configuration. Applets are only allowed to create sockets back to the same server that they came from. While this is probably overkill for the home user, there is a very good reason for this policy: Firewalls. In the corporate world, most Internet security is established through the use of firewalls, which restrict incoming connection requests. A Java applet that could connect anywhere could establish a link that allowed hacker.org to telnet in to payroll.largecomp.com. Since even HTTP-GET connections can pass along sensitive information (http://www.hacker.org/dummy.cgi?Bob+has+no+password), all connections have to be restricted. The security manager is apparently configurable in Netscape 4.0, but I've had little luck in getting it to work. [Note: I should really have a better answer here for version 4 browsers!] For a good discussion of applet security issues, see http://www.innovation.ch/java/HTTPClient/security.html