Using the top-to-bottom OSI model approach to troubleshoot the issue, the first step you should take is to ensure that the IP addresses are correctly assigned to the marketing department's computers. This action falls under the Network layer (Layer 3) of the OSI model, where IP addressing and routing are managed. Since this issue is isolated to a specific department, there may be a problem related to network configuration or addressing for that department’s devices.
Once you've confirmed that the IP addresses are correct, you can move down the OSI model as needed, checking for physical connections (Layer 1), data link issues (Layer 2), and application-related problems (Layer 7) as you further diagnose the issue. Starting with IP addresses is a logical first step before moving on to other potential causes.