Access Control List Fundamentals / Introduction to ACLs

Access Control List Fundamentals 
Introduction to ACLs
11.1.1 This page will explain what ACLs are and how they are used.

ACLs are lists of conditions used to test network traffic that tries to travel across a router interface. These lists tell the router what types of packets to accept or deny. Acceptance and denial can be based on specified conditions. ACLs enable management of traffic and secure access to and from a network.

ACLs can be created for all routed network protocols such as IP and Internetwork Packet Exchange (IPX). ACLs can be configured at the router to control access to a network or subnet.

To filter network traffic, ACLs determine if routed packets are forwarded or blocked at the router interfaces. The router examines each packet and will forward or discard it based on the conditions specified in the ACL. An ACL makes routing decisions based on source address, destination address, protocols, and upper-layer port numbers.

ACLs must be defined on a per protocol, per direction, or per port basis. To control traffic flow on an interface, an ACL must be defined for each protocol enabled on the interface. ACLs control traffic in one direction at a time on an interface. Two separate ACLs must be created to control inbound and outbound traffic. Every interface can have multiple protocols and directions defined. If the router has two interfaces configured for IP, AppleTalk, and IPX, 12 separate ACLs would be needed. There would be one ACL for each protocol, times two for each direction, times two for the number of ports.

ACLs can be used to perform the following tasks:

• Limit network traffic and increase network performance. For example, ACLs that restrict video traffic could greatly reduce the network load and increase network performance.
• Provide traffic flow control. ACLs can restrict the delivery of routing updates. If updates are not required because of network conditions, bandwidth is preserved.
• Provide a basic level of security for network access. ACLs can allow one host to access a part of the network and prevent another host from accessing the same area. For example, Host A is allowed to access the Human Resources network and Host B is prevented from accessing it.
• Decide which types of traffic are forwarded or blocked at the router interfaces. ACLs can permit e-mail traffic to be routed, but block all Telnet traffic.
• Control which areas a client can access on a network.
• Screen hosts to permit or deny access to a network segment. ACLs can be used to permit or deny a user to access file types such as FTP or HTTP.

If ACLs are not configured on the router, all packets that pass through the router will be permitted to access the entire network.
The next page will describe how ACLs work.