We have built an Office Web Add-in for Excel, which renders a Web Application inside Excel Task pane.
The Web Application is hosted on-premises on IIS/ASP.NET.
We are able to launch the add-in within Excel desktop.
What network infrastructure / hosting is required to launch the add-in within Office 365 Excel online in a way that only allows users within the organisation to access the Web Application - would it require public IP?
From an authentication standpoint, we can restrict access to Web Application using SSO with our Azure AD tenant.
The add-in will be deployed and managed through centralized deployment.
Office add-ins don't introduce any restrictions where and how to host the application. You just need to address the same-origin policy, read more about that in the Addressing same-origin policy limitations in Office Add-ins article.