How to create and configure Standard Access Control Lists, access-list IOS command and access-group IOS command

Before continuing, refer Introduction to Access Control Lists lesson , if you are not familiar with Access Contol Lists.

Refer Standard Access Control Lists lesson if you are not familiar not familiar with Standard Access Control configuration IOS commands.

Standard Access Control Lists (ACLs) - Lab Practice

The following diagram shows our Standard Access Control Lists lab setup. We have three routers, three switches, six workstations and three servers connected as below. The host names, IP addresses and the interfaces of the routers are shown in diagram. The IP addresses of the workstations and the servers are also shown in the diagram.

The purpose of this lab is to deny the workstations located at 172.16.0.0/16 network from accessing the servers at 172.20.0.0/16 network, using Standard Access Control List. Connect computers, switches and routers as shown in the figure. Configure the IP address and default gateway TCP/IP settings in all computers and servers. Configure the hostname, IP address and routing on three routers. Click the following link to learn more about configuring hostnames, IP addresses and Enhanced Interior Gateway Routing Protocol (EIGRP) in routers.

Click the following link to know where to place a Standard Access Control List (ACL). In this lab, the router near to the destination network is Router03, and we have to configure Standard Access Control List (ACL) in Router03.

How to create Standard Access Control List (ACL) using "access-list" IOS command

To create a Standard Access Control List (ACL), to deny all the IP addresses from 172.16.0.0/16 network, from accessing the servers at 172.20.0.0/16 network, we use the "access-list" IOS command from the global configuration mode of Router03 (which is close to the destination), as shown below.

Router03>enable
Router03#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router03(config)#access-list 5 deny 172.16.0.0 0.0.255.255
Router03(config)#access-list 5 permit any
Router03(config)#exit
Router03#

Remember, there is an implicit "deny any" at the end of every Access Control Lists (ACL). If there is no "access-list 5 permit any" statement at the end, above Standard Access Control Lists (ACL) may filter out all traffic to the destination network. The "access-list 5 permit any" permits any other traffic, if there is no matching deny in previous statements. Above Standard Access Control Lists (ACL) effectively allow all the traffic to the destination network except 172.16.0.0/16 network.

If you want to remove the Access Control List (ACL), use the "no" form of the command. You cannot delete a specific entry in an Access Control List (ACL). You can only delete the entire Access Control List (ACL), as shown below.

Router03(config)#no access-list 5 

How to configure Standard Access Control Lists (ACL) to an interface using "access-group" command

The "access-group" command can be used to apply the access list to an interface. The syntax for "access-group" IOS command is given below.

Router(config)# interface interface_no
Router01(config-if)#ip access-group <access_list_number> <in/out>

The "in/out" keyword of the command is used to specify the direction in which the traffic is filtered.

The "in" keyword is used to specify that the traffic should be filtered when it arrive the router via an interface. Following diagram explains the "in" keyword. Here the traffic will be filtered as it arrives the router.

The "out" keyword is used to specify that the traffic should be filtered as it leaves the router via an interface. Following diagram explains the "out" keyword. Here the traffic will be filtered as it leaves the router.

 

The Standard Access Control Lists must be applied close to the destination network. Here the interface close to the destination is fa0/0 in Router03. Following IOS commands apply the Standard Access Control List (Access Control List No. 5) to the interface fa0/0 (Router03) in "out" direction.

Router03>enable
Router03#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router03(config)#interface fa0/0
Router03(config-if)#ip access-group 5 out
Router03(config-if)#exit
Router03(config)#exit
Router03#

If you want to remove the Access Control List (ACL) from the above interface, use the "no" form of the command as shown below.

 Router01(config-if)#no ip access-group 5 out