At which layer of the OSI model does a packet filter operate?

A packet filter operates at Layer 3 of the OSI model, which is the Network layer. This layer is responsible for routing packets through the network based on their IP addresses. In particular, packet filters examine the header information of the packets, allowing or blocking them based on predefined rules related to IP addresses, protocols, and port numbers. By functioning at this layer, packet filters can effectively manage network traffic and enforce security policies regarding what kinds of data can enter or leave a network.

The other layers listed do not handle packet filtering as these functions belong to Layer 3. Layer 1 deals with the physical aspects of networking, such as cables and switches; Layer 5, known as the Session layer, manages sessions and connections; and Layer 7, the Application layer, deals with application-level interactions and data formats. Hence, the focus of packet filtering being at Layer 3 aligns with its role in ensuring packet-level security and routing in network communications.