Given that the users can communicate with devices within the LAN but not with external IP addresses, the issue seems to be with routing. This function takes place at Layer 3 of the OSI model, the Network layer. The Network layer is responsible for determining the best path for data to travel from the source to the destination using IP addresses. A possible cause might be an incorrect or missing route in the routing table, preventing packets from leaving the LAN. Problems with connectors and cabling (Layer 1) or switches operating at Layer 2 would typically not allow successful pinging of devices even within the LAN if they were the cause. Similarly, application configuration (Layer 7) would not affect the ability to ping external IP addresses, as that is a network-level issue, not an application-level one.