Is an Azure app service plan analogous to an instance of IIS?

Question

I've been struggling with adding a custom domain to one of my sites because it's already in use on another site. I have these:

TMProfile1                         tm profile
 +EastUSAppServicePlan1            app service plan S2
   +eus-x-com.azurewebsites.net    app service
     +x.com                        custom domain

 +WestUSAppServicePlan1            app service plan S2
   +wus-x-com.azurewebsites.net    app service
     +x.com                        custom domain 

These sites both have a custom domain of x.com set up. Both these sites are in the same traffic manager profile and the DNS points x.com -CNAME-> tmprofile1.trafficmanager.net . All is good

I've created another app service plan (this time an S1) called EastUsAppServicePlan2 and I created a new app service inside of it called eus2-x-com.azurewebsites.net . Even though this new app service is in a different app service plan to the two that already exist, I'm trying to add the same x.com custom domain to this new app service:

x.com custom domain is already assigned to website eus-x-com.azurewebsites.net

I always thought that an app service plan is analogous to an instance of IIS, and that every app service plan would have a unique IP address but it seems not. If I nslookup eus-x-com.azurewebsites.net (a site in EastUsAppServicePlan1) I get an IP of 12.34.56.78 and if I nslookup eus2-x-com.azurewebsites.net (the site in the new EastUSAppServicePlan2) I get the same IP of 12.34.56.78

It seems to be then, that it is region that determines IP; even if I create different app service plans in the same region, they will all have the same IP address? (It might explain some of the other issues I'm having)

Answer 1

By pulling the definition from the Microsoft documentation , we have:

Azure App Service is a multi-tenant service, except for App Service Environments. Apps that are not in an App Service environment (not in the Isolated tier) share network infrastructure with other apps. As a result, the inbound and outbound IP addresses of an app can be different, and can even change in certain situations.

So, even in same region you may have scenarios that your apps, using different service plans, have same inbound ip addresses, or different. That's managed by Azure, so you have no control over it, unless you have a scenario that you want to use a dedicated static inboud ip address to your apps, then you can leverage the IP-based SSL binding feature to have the same ip address for them.