In the context of IoT communication models, the Device-to-Device (D2D) model refers to the direct interaction between devices without the need for intermediary devices or services. This model is characterized by the use of protocols such as ZigBee, Z-Wave, or Bluetooth, which are designed to facilitate direct communication between devices in close proximity. These protocols are commonly used in home automation, where devices like sensors, lights, and locks need to communicate with each other to perform their functions effectively.