Definition of an Access List
Access control lists (ACLs) perform packet filtering to control the movement of packets through a network. Packet filtering provides security by limiting the access of traffic into a network, restricting user and device access to a network, and preventing traffic from leaving a network. IP access lists reduce the chance of spoofing and denial-of-service attacks, and allow dynamic, temporary user-access through a firewall.
IP access lists can also be used for purposes other than security, such as to control bandwidth, restrict the content of routing updates, redistribute routes, trigger dial-on-demand (DDR) calls, limit debug output, and identify or classify traffic for quality of service (QoS) features.
An access list is a sequential list that consists of at least one permit statement and possibly one or more deny statements. In the case of IP access lists, these statements can apply to IP addresses, upper-layer IP protocols, or other fields in IP packets.
Access lists are identified and referenced by a name or a number. Access lists act as packet filters, filtering packets based on the criteria defined in each access list.
After you configure an access list, for the access list to take effect, you must either apply the access list to an interface (by using the ip access-group command), a vty (by using the access-class command), or reference the access list by any command that accepts an access list. Multiple commands can reference the same access list.
In the following configuration, an IP access list named branchoffices is configured on Fast Ethernet interface 0/1/0 and applied to incoming packets. Networks other than the ones specified by the source address and mask pair cannot access Fast Ethernet interface 0/1/0. The destinations for packets coming from sources on network 172.16.7.0 are unrestricted. The destination for packets coming from sources on network 172.16.2.0 must be 172.31.5.4.
ip access-list extended branchoffices 10 permit 172.16.7.0 0.0.0.3 any 20 permit 172.16.2.0 0.0.0.255 host 172.31.5.4 ! interface fastethernet 0/1/0 ip access-group branchoffices in