ACLs are lists of conditions that are applied to traffic traveling across a router's 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 Internet Protocol (IP) and Internetwork Packet Exchange (IPX). ACLs can be configured at the router to control access to a network or subnet.

ACLs filter network traffic by controlling whether routed packets are forwarded or blocked at the router's interfaces. The router examines each packet to determine whether to forward or drop it, based on the conditions specified in the ACL. Some ACL decision points are source and destination addresses, 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. A separate ACL would need to be created for each direction, one for inbound and one for outbound traffic. Finally 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. One ACL for each protocol, times two for direction in and out, times two for the number of ports.

The following are some of the primary reasons to create ACLs:

• Limit network traffic and increase network performance. By restricting video traffic, for example, ACLs could greatly reduce the network load and consequently 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. Permit e-mail traffic to be routed, but block all telnet traffic.
• Allow an administrator to control what areas a client can access on a network.
• Screen certain hosts to either allow or deny access to part of a network. Grant or deny user permission to access only certain types of files, such as FTP or HTTP.

If ACLs are not configured on the router, all packets passing through the router will be allowed onto all parts of the network.

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