ACL (Access Control List)

The ACL rules are ordered, and the first rule that matches a packet is the one that is applied. If no rule matches the packet, then the packet is denied by default.

Here is an example of an ACL rule:

10 permit tcp any any eq 80

This rule allows any traffic on port 80 (HTTP) from any source to any destination.

ACLs can be used to control a wide variety of traffic on a router, including inbound, outbound, and local traffic. They can also be used to control traffic based on protocol, source and destination IP addresses, source and destination ports, and other criteria.

ACLs are a powerful tool for network security, and they can be used to protect networks from a variety of threats. However, it is important to use ACLs carefully, as they can also be used to unintentionally block legitimate traffic.

The structure of an access control list (ACL) is as follows:

• Rule number: This is a unique identifier for the ACL rule.
• Protocol: This specifies the type of traffic that the rule applies to. For example, TCP, UDP, or ICMP.
• Source address: This specifies the source IP address or network that the traffic is coming from.
• Destination address: This specifies the destination IP address or network that the traffic is going to.
• Source port: This specifies the source port number, if applicable.
• Destination port: This specifies the destination port number, if applicable.
• Action: This specifies whether the traffic is permitted or denied.
• Comment: This is an optional field that can be used to provide additional information about the ACL rule.