What command is used to apply an ACL to an interface?

The command used to apply an Access Control List (ACL) to an interface is "ip access-group acl {in | out}". This command is entered in the interface configuration mode and specifies that packets matching the defined ACL should be processed either as they enter the interface (in) or as they exit the interface (out).

When using this command, you associate a specific ACL with an interface, enabling the router or switch to enforce the defined filtering rules on traffic according to the ACL's criteria. This application is crucial for controlling network access and working towards security and traffic management.

Other options, while related to ACLs, do not serve the specific purpose of applying an ACL to an interface. For example, "access-list number {in | out}" refers to the command for creating ACL entries themselves rather than applying them. Similarly, "ip access-list standard acl-name" is used to define a named ACL, and "configure access-group acl {in | out}" is not a valid command in Cisco IOS configuration. Thus, "ip access-group acl {in | out}" is the correct and specific command for associating an ACL with an interface.