Access List Tutorial

Named IP Access List

This allows standard and extended ACLs to be given names instead of numbers

Named IP Access List Configuration Syntax

Example of Named IP Access List

This is an example of the use of a named ACL in order to block all traffic except the Telnet connection from host 10.0.0.1/8 to host 187.100.1.6.

Define the ACL:

Router(config)#ip access-list extended in_to_out permit tcp host 10.0.0.1 host 187.100.1.6 eq telnet

(notice that we can use ‘telnet’ instead of port 23)

Apply this ACL to an interface:

Router(config)#interface Fa0/0

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

Where to place access list?

Standard IP access list should be placed close to destination.

Extended IP access lists should be placed close to the source.

How many access lists can be used?

You can have one access-list per protocol, per direction and per interface. For example, you can not have two access lists on the inbound direction of Fa0/0 interface. However you can have one inbound and one outbound access list applied on Fa0/0.

How to use the wildcard mask?

Wildcard masks are used with access lists to specify a host, network or part of a network.

The zeros and ones in a wildcard determine whether the corresponding bits in the IP address should be checked or ignored for ACL purposes. For example, we want to create a standard ACL which will only allow network 172.23.16.0/20 to pass through. We need to write an ACL, something like this:

access-list 1 permit 172.23.16.0 255.255.240.0

Of course we can’t write subnet mask in an ACL, we must convert it into wildcard mask by converting all bits 0 to 1 & all bits 1 to 0.

255 = 1111 1111 -> convert into 0000 0000

240 = 1111 0000 -> convert into 0000 1111

0 = 0000 0000 -> convert into 1111 1111

Therefore 255.255.240.0 can be written in wildcard mask as 00000000.00000000.00001111.11111111 = 0.0.15.255

Remember, for the wildcard mask, 1′s are I DON’T CARE, and 0′s are I CARE. Now let’s analyze our wildcard mask.

Two first octets are all 0’s meaning that we care about the network 172.23.x.x. The third octet, 15 (0000 1111 in binary), means that we care about first 4 bits but don’t care about last 4 bits so we allow the third octet in the form of 0001xxxx (minimum:00010000 = 16; maximum: 00011111 = 31).

The fourth octet is 255 (all 1 bits) that means I don’t care.

Therefore network 172.23.16.0 0.0.15.255 ranges from 172.23.16.0 to 172.23.31.255.

Some additional examples:

+ Block TCP packets on port 30 from any source to any destination:

Router(config)#access-list 101 deny tcp any any eq 30

+ Permit any IP packets in network 192.23.130.128 with subnet mask 255.255.255.248 to any network:

Router(config)#access-list 101 permit ip 192.23.130.128 0.0.0.7 any

Apply the access control list to an interface:

Router(config)#interface fastEthernet0/0

Router(config-if)#ip access-group 101 in

Note: An ACL applied to the main interface does not affect the traffic of subinterfaces. If we want to filter traffic on subinterfaces, we have to assign ACL to each subinterface separately.