| 11.2 |  | Access Control Lists (ACLs) |
| 11.2.6 |  | Restricting virtual terminal access |

| Standard and extended access lists apply to packets traveling through a router. They are not designed to block packets that originate within the router. An outbound Telnet extended access list does not prevent router initiated Telnet sessions, by default. Just as there are physical ports or interfaces, such as Fa0/0 and S0/0 on the router, there are also virtual ports. These virtual ports are called vty lines. There are five such vty lines, numbered 0 through 4, as shown in figure . For security purposes, users can be denied or permitted virtual terminal access to the router but denied access to destinations from that router. The purpose of restricted vty access is increased network security. Access to vty is also accomplished using the Telnet protocol to make a nonphysical connection to the router. As a result, there is only one type of vty access list. Identical restrictions should be placed on all vty lines as it is not possible to control which line a user will connect on. The process to create the vty access list is the same as described for an interface. However, applying the ACL to a terminal line requires the access-class command instead of the access-group command. The following should be considered when configuring access lists on vty lines: When controlling access to an interface, a name or number can be used. Only numbered access lists can be applied to virtual lines. Set identical restrictions on all the virtual terminal lines, because a user can attempt to connect to any of them.
| Lab Activity e-Lab Activity: Access Control Lists In this lab, the students will practice using ACLs to filter IP traffic.
| Web Links Strategies & Issues: Ports of Entry - Routers in the Crosshairs http://www.networkmagazine.com/shared/article/ showArticle.jhtml?articleId=8703354&classroom=