# ACL Placement ACL Placement is the strategic process of applying Access Control Lists (ACLs) on a network device's interfaces to control traffic flow. The primary goal is to filter unwanted traffic as efficiently and securely as possible, typically by blocking it as close to its source as the ACL's specificity allows, thereby conserving network resources. **Pronunciation:** A-C-L Place-ment **Difficulty:** Intermediate **Synonyms:** ACL Application, Access List Placement, ACL Strategy **Categories:** Network Security, Network Administration **Tags:** networking, security, cisco, firewall, traffic filtering Canonical: https://scaleengineer.com/glossaries/acl-placement --- ## Definitions - **Core Principles of ACL Placement:** The primary goal of **ACL Placement** is to filter traffic efficiently without disrupting legitimate network services. The strategy depends heavily on the type of ACL being used. *** **Standard ACLs** Standard ACLs filter traffic based solely on the source IP address. Because they are not very specific, they can easily cause unintended consequences if placed incorrectly. * **Rule**: Place as close to the **destination** as possible. * **Reasoning**: Placing a standard ACL near the source can block that source from accessing all other network resources, not just the intended target. For example, if you want to block Host A from reaching Server X, applying a standard ACL at the router nearest to Host A would also block its access to Server Y and Server Z. By placing it on the router interface closest to Server X, you only affect traffic destined for that specific server's network segment. *** **Extended ACLs** Extended ACLs are much more granular, allowing filtering based on source IP, destination IP, protocol (TCP/UDP), and source/destination port numbers. This specificity is key to their placement strategy. * **Rule**: Place as close to the **source** as possible. * **Reasoning**: Since an extended ACL can precisely target specific traffic (e.g., block Host A from accessing the web server on port 80 of Server X), it can be safely applied at the first router the traffic encounters. This is the most efficient **ACL Application** because it drops unwanted traffic immediately, preventing it from consuming bandwidth and processing power across the network infrastructure. *** **Directional Application** ACLs are applied to an interface in a specific direction: * **Inbound (ingress)**: The ACL processes packets as they *enter* the interface, before they are routed to an outbound interface. * **Outbound (egress)**: The ACL processes packets just before they *leave* the interface. The choice of direction is a critical part of **ACL Placement**. To filter traffic from a source network, it is most common to apply the ACL in the `in` direction on the interface that receives traffic from that network. ## Etymology The term is a compound of 'ACL' (an acronym for Access Control List) and 'Placement' (the act of positioning something). It literally means the strategic positioning of an Access Control List within a network topology. ## First used 1990s ## Historical context Access Control Lists (ACLs) emerged with early routing hardware as a fundamental tool for managing network traffic. Initially, their use was for basic traffic control and to enforce simple routing policies. As networks, particularly the internet, grew exponentially in the 1990s, the need for more robust security measures became paramount. This shift from simple control to security enforcement gave rise to the formal concept of **ACL Placement**. Network engineers realized that where an ACL was applied was just as important as what it contained. Vendors like Cisco were instrumental in popularizing these best practices through their training and certification programs (e.g., CCNA), which emphasized the distinction between standard and extended ACLs. The development of extended ACLs, which could filter on more than just the source address, was a critical turning point. It enabled a more efficient **ACL Strategy**: filtering unwanted traffic close to the source. This prevented malicious or unnecessary packets from ever entering the core network, saving valuable bandwidth and router processing power. The principles of **Access List Placement** thus became a cornerstone of effective and efficient network security design. ## Q&A - **What is the general rule of thumb for placing standard and extended ACLs?:** The general best practice is to place **standard ACLs** as close to the destination as possible. Because standard ACLs can only filter based on the source IP address, placing them near the source might inadvertently block that source from accessing other necessary resources. Conversely, **extended ACLs** should be placed as close to the source as possible. Since extended ACLs are highly specific (filtering on source/destination IP, protocol, and port numbers), they can precisely deny unwanted traffic at its entry point to the network, saving bandwidth and processing power on core devices. - **Why is it inefficient to place an extended ACL that blocks a specific server's web traffic close to the server itself?:** Placing an extended ACL close to the destination server is inefficient because the unwanted traffic has already traversed the entire network to reach that point. This consumes bandwidth on all intermediate links and processing cycles on all routers along the path. The core principle of **ACL Placement** is to drop unwanted traffic as early as possible (close to the source) to conserve network resources. This is a key reason why the **ACL Strategy** for extended lists is to apply them near the traffic's origin. ## Usage examples - The network administrator carefully planned the **ACL Placement** to ensure that unwanted traffic from the guest Wi-Fi was dropped at the edge router, preventing it from consuming core bandwidth. - A poor **ACL Application** strategy can lead to security holes or accidentally block critical services, so it's crucial to test changes in a lab environment first. - For our new security policy, the **Access List Placement** for extended ACLs will be on the ingress interfaces of our distribution layer switches, as close to the user subnets as possible. ## Related terms - Standard ACL - Extended ACL - Named ACL - Numbered ACL - Ingress Filtering - Egress Filtering - Firewall Rules ## Popular related terms - Standard ACL - Extended ACL - Firewall - Ingress Filtering - Network Security