IP Access Control Lists

IP Access Control Lists (ACLs) are used in network security to filter and control IP traffic based on source and destination IP addresses, protocols, and ports. ACLs can be applied to interfaces on routers, switches, and firewalls to control the flow of traffic through the network.

ACLs can be configured in two ways: standard and extended. Standard ACLs can filter traffic based only on the source IP address, while extended ACLs can filter traffic based on source and destination IP addresses, protocols, and ports.

ACLs are also divided into two directions: inbound and outbound. Inbound ACLs are applied to incoming traffic on an interface, while outbound ACLs are applied to outgoing traffic on an interface.

The syntax for configuring ACLs varies depending on the device and the type of ACL being used. Here are some examples:

Configuring a standard ACL to block traffic from a specific IP address on a Cisco router:

Router(config)# access-list 10 deny host 192.168.1.100

Router(config)# access-list 10 permit any

Router(config)# interface GigabitEthernet0/0

Router(config-if)# ip access-group 10 in

In this example, ACL 10 is configured to deny traffic from the IP address 192.168.1.100 and permit all other traffic. The ACL is then applied inbound to the GigabitEthernet0/0 interface.

Configuring an extended ACL to allow only SSH traffic from a specific subnet on a Cisco firewall:

ASA(config)# access-list 101 permit tcp 10.1.1.0 255.255.255.0 any eq ssh

ASA(config)# access-list 101 deny ip any any

ASA(config)# interface GigabitEthernet0/0

ASA(config-if)# access-group 101 in

In this example, ACL 101 is configured to permit only SSH traffic from the 10.1.1.0/24 subnet to any destination, and to deny all other traffic. The ACL is then applied inbound to the GigabitEthernet0/0 interface on the firewall.

It is important to carefully plan and test ACL configurations to ensure that they do not inadvertently block legitimate traffic or create security vulnerabilities. In addition, ACLs should be regularly reviewed and updated as network requirements and threats evolve.