Abstract: The access control list controls the access between internal and external networks through a pre-established sequence of commands, and is widely used in the environment of internal and external network isolation. Two different types of network topologies was designed by using the Packet Tracer and GNS 3 software. Through device configuration and further application of access control lists, the purpose of isolation between internal and external networks was achieved. By analyzing the function mechanism of the access control list in the isolated intranet and extranet networks, the further deepen of the understanding and mastery of the access control list technology was achieved, and at the same time, it has certain reference significance for the isolation of large-scale campus networks.