IP filtering is simply a mechanism that decides which types of
IP datagrams will be processed normally and which will be
discarded. By discarded we mean that the datagram is deleted and
completely ignored, as if it had never been received. You can
apply many different sorts of criteria to determine which
datagrams you wish to filter; some examples of these are:
Protocol type: TCP, UDP, ICMP, etc. Socket number (for TCP/UPD)
Datagram type: SYN/ACK, data, ICMP Echo Request, etc. Datagram
source address: where it came from Datagram destination address:
where it is going to It is important to understand at this point
that IP filtering is a network layer facility. This means it
doesn't understand anything about the application using the
network connections, only about the connections themselves. For
example, you may deny users access to your internal network on
the default telnet port, but if you rely on IP filtering alone,
you can't stop them from using the telnet program with a port
that you do allow to pass trh