If the Office Web Apps Server is installed on-premises and in the same network zone as Skype for Business Server then the option Office Web Apps Server is deployed in an external network (that is, perimeter/Internet) shouldn't be selected. In the Define New Office Web Apps Server dialog box, type the fully qualified domain name (FQDN) of your Office Web Apps Server computer in the Office Web Apps Server FQDN box when you do this, your Office Web Apps Server discovery URL should automatically be entered into the Office Web Apps Server discovery URL box. In the Edit Properties dialog box, on the General tab, find the heading Associate Office Web Apps Server and then click New (or select an existing Office Web Apps Server from the drop-down list). In Topology Builder, expand Skype for Business Server, expand the name of your site, expand Enterprise Edition Front End pools, right-click the name of one of your pools, and then click Edit Properties. This topology can later be retrieved and republished if you encounter problems with your new topology. In the Save Topology As dialog box, type a name for your topology document (for example, PreWebAppsServerTopology) in the File name box and then click Save. In the Topology Builder dialog box, select Download Topology from existing deployment and then click OK. Open Skype for Business Server Topology Builder.

To add Office Web Apps Server to your topology, complete the following steps: Configure Skype for Business Server to communicate with Office Web Apps Server

For more detail, refer to the Office Online Server documentation. The latest iteration of Office Web Apps Server is named Office Online Server, which is supported by Skype for Business Server.

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