Packet filtering operates at the network layer of the Open System Interconnection (OSI) model. The OSI model is a conceptual framework that describes how data is transmitted and processed across different layers of a network. The OSI model consists of seven layers: application, presentation, session, transport, network, data link, and physical. The network layer is the third layer from the bottom of the OSI model, and it is responsible for routing and forwarding data packets between different networks or subnets. The network layer uses logical addresses, such as IP addresses, to identify the source and destination of the data packets, and it uses protocols, such as IP, ICMP, or ARP, to perform the routing and forwarding functions.

Packet filtering is a technique that controls the access to a network or a host by inspecting the incoming and outgoing data packets and applying a set of rules or policies to allow or deny them. Packet filtering can be performed by devices, such as routers, firewalls, or proxies, that operate at the network layer of the OSI model. Packet filtering typically examines the network layer header of the data packets, such as the source and destination IP addresses, the protocol type, or the fragmentation flags, and compares them with the predefined rules or policies. Packet filtering can also examine the transport layer header of the data packets, such as the source and destination port numbers, the TCP flags, or the sequence numbers, and compare them with the rules or policies. Packet filtering can provide a basic level of security and performance for a network or a host, but it also has some limitations, such as the inability to inspect the payload or the content of the data packets, the vulnerability to spoofing or fragmentation attacks, or the complexity and maintenance of the rules or policies.

The other options are not techniques that operate at the network layer of the OSI model, but rather at other layers. Port services filtering is a technique that controls the access to a network or a host by inspecting the transport layer header of the data packets and applying a set of rules or policies to allow or deny them based on the port numbers or the services. Port services filtering operates at the transport layer of the OSI model, which is the fourth layer from the bottom. Content filtering is a technique that controls the access to a network or a host by inspecting the application layer payload or the content of the data packets and applying a set of rules or policies to allow or deny them based on the keywords, URLs, file types, or other criteria. Content filtering operates at the application layer of the OSI model, which is the seventh and the topmost layer. Application access control is a technique that controls the access to a network or a host by inspecting the application layer identity or the credentials of the users or the processes and applying a set of rules or policies to allow or deny them based on the roles, permissions, or other attributes. Application access control operates at the application layer of the OSI model, which is the seventh and the topmost layer.