7.1 Forwarding and Screening
Forwarding enables the use of a station (for example, computer) as a router. When a station finds out that an IP datagram is not addressed to it, it tries to forward it, i.e., send it in the same way as it sends its own IP datagrams.
Forwarding can be banned too, by configuring the kernel of the operating system. With older systems, such a ban required a recompilation of the kernel of the operating system. With current systems, the same can be done dynamically (for example, Windows 2000, Windows XP, or most UNIX systems). It may, however, be necessary to restart the system after such modification.
The situation with Windows 2000/XP is interesting. In Windows 2000/XP, forwarding is set by inserting the value 1 into the IpEnableRouter key, which is in the HKEY_LOCAL_MACHINE\SYSTEM\ CurrentControlSet\Services\Tcpip\Parameters registry folder.
It is an interesting feature of many operating systems that they do not forward IP datagrams mechanically, but screen them...
