Basic Networking

  What is an ACL? An ACL is a list of permit or deny rules detailing what can or can't enter or leave the interface of a router. Every packet that attempts to enter or leave a router must be tested against each rule in the ACL until a match is found. If no match is found, then it will be denied. To get a bit more technical, when a packet is sent out, it must know where it's going (destination) and where it came from (source). So it contains a source and destination IP address. The router looks at this information to determine if it matches any of the rules in its ACL. If a router can't find a match between the information in an ACL and the information in the packet that's attempting to enter it, the packet is denied implicitly. How Does Implicit Deny Work? The last rule in every ACL is an implicit deny statement. Because it's implicit, you won't see it. Be aware that just because you don't see it doesn't mean it doesn't do anything. This rule is very...