A packet-filtering router usually depends on access rules—that is, rules you set up within the router software itself. A packet filter usually has a rule set that starts with least common and works its way up to most common. What’s a rule set? Typically, a rule set looks something like a routing table but includes ports as well as addresses. Any packet that comes in is compared against rule 1, and then rule 2, all the way to the end. If at any time it matches up against a rule, processing stops. For example, for my 192.168.1.0 network, the rules might be as follows:
Allow 192.168.1.0:any on interface 0 to connect to all:any.
Deny all:any to connect to all:any.
This means that anybody within my 192.168.1.0 network (provided ...