Access Control List (ACL) Deployment Guide

1. What is an ACL?

An Access Control List (ACL) is a sequential collection of permit or deny statements applied to an interface. It filters network traffic based on parameters like source IP, destination IP, or port numbers, serving as a primary line of defense in network architecture.

2. Standard vs. Extended ACLs

Choosing the wrong type of list can accidentally choke your entire network segment. Remember these rules:

• Standard ACLs (1-99): Only inspect the source IP address. Place them as close to the destination as possible.
• Extended ACLs (100-199): Inspect source, destination, protocol types, and layer 4 port numbers. Place them as close to the source as possible.

Important: Every ACL has an invisible statement at the absolute bottom: deny any. If traffic doesn't explicitly match a permit rule, it gets dropped immediately.