List kept by routers to control access to or from the router for a number of services. For example, to prevent packets with a certain IP address from leaving a particular interface on the router. Generally the way in which network devices access the network medium.

Reasons for create ACL:

1.   Limit network traffic and increase network performance.

2.   Provide traffic flow control.

3.   Provide a basic level of security for network access.

4.   Decide which types of traffic are forwarded or blocked at the router interfaces.

5.   Restrict Telnet (VTY) access to a router.

6.   ACL’s commands define specifically which traffic is permitted and which is denied.

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