Access Control Lists (ACLs)
Access Control List (ACL) are filters that enable you to control which routing updates or packets are permitted or denied in or out of a network. They are specifically used by network administrators to filter traffic and to provide extra security for their networks. This can be applied on routers (Cisco).
ACLs provide a powerful way to control traffic into and out of your network; this control can be as simple as permitting or denying network hosts or addresses. You can configure ACLs for all routed network protocols.
The most important reason to configure ACLs is to provide security for your network. However, ACLs can also be configured to control network traffic based on the TCP port being used.
How ACL works
A router acts as a packet filter when it forwards or denies packets according to filtering rules. As a Layer 3 device, a packet-filtering router uses rules to determine whether to permit or deny traffic based on source and destination IP addresses, source port and destination port, and the protocol of the packet. These rules are defined using access control lists or ACLs.
To simplify how ACL or a router uses packet filtering work, imagine that a guard has been posted at a locked door. The guard's instructions are to allow only people whose names appear on a list to pass through the door. The guard is filtering people based on the condition of having their names on the authorized list.
When a packet arrives at the router, the router extracts certain information from the packet header and makes decisions according to the filter rules as to whether the packet can pass through or be dropped. Packet filtering process works at the Network layer of the Open Systems Interconnection (OSI) model, or the Internet layer of TCP/IP.
Why use ACLs
* Limits network traffic to increase network performance.
* ACLs provides traffic flow control by restricting the delivery of routing updates.
* It can be used as additional security.
* Controls which type of traffic are forwarded or blocked by the router.
* Ability to control which areas a client access.
Types of Access Control Lists
Standard access-list
Standard access lists create filters based on source addresses and are used for server based filtering. Address based access lists distinguish routes on a network you want to control by using network address number (IP). Address-based access lists consist of a list of addresses or address ranges and a statement as to whether access to or from that address is permitted or denied.
Extended access lists
Extended access lists create filters based on source addresses, destination addresses, protocol, port number and other features and are used for packet based filtering for packets that traverse the network.
Standard and Extended access lists can be applied base on the use of ip access-list command.
Access lists use the deny or permit statement to define which packet is allowed or denied entry into a server or network.
Masks
Masks are used with IP addresses in IP ACLs to specify what should be permitted and denied. Masks in order to configure IP addresses on interfaces start with 255 and have the large values on the left side, for example, IP address 172.16.2.14 with a 255.255.255.0 mask. Masks for IP ACLs are the reverse, for example, mask 0.0.0.255. This is sometimes called an inverse mask or a wildcard mask. When the value of the mask is broken down into binary (0s and 1s), the results determine which address bits are to be considered in processing the traffic. A 0 indicates that the address bits must be considered (exact match); a 1 in the mask is a "no".
Note these ACL equivalents.
· The source/source-wildcard of 0.0.0.0/255.255.255.255 means "any".
· The source/wildcard of 10.1.1.2/0.0.0.0 is the same as "host 10.1.1.2".
If you subtract 255.255.255.0 (normal mask) from 255.255.255.255, it yields 0.0.0.255.
Read about Wildcards
The command below defines an ACL that permits this network 192.168.1.0 0.0.0.255.
access-list acl_permit permit ip 192.168.1.0 0.0.0.255
Inbound traffic to the router is compared to access lists entries based on the order that the entries occur in the router. The router looks through the entries until it has a match. If the router found no match when it reaches the end of the list, the traffic is denied. For this reason, you should have the frequently hit entries at the top of the list. There is an implied deny for traffic that is not permitted. Single-entry access lists with only one deny entry has the effect of denying all traffic. You must have at least one permit statement in an ACL or all traffic is blocked.
Access lists implicitly deny all access that is not expressly permitted. The following line is auto-appended to all access-lists:
deny ip any any
If it is desirable to over-ride this implicit denial statement, enter a permit ip any any statement as the last entry in the access-list. | 
