Building networks in the cloud is sometimes hard to understand. A common mistake is to believe that all VMs can talk to another, regardless of the owner, and that all VMs are available over the internet.
Some basics about Cloud Service Endpoints and Virtual Networks
When we talk about Microsoft Azure, a Cloud Service Endpoint is the easiest way to access one or multiple VMs. A Cloud Service contains resources, like VMs, and it’s acting as a communication and security boundary. All VMs that use the same Cloud Service get their IPs via DHCP and share the same private IP address range. The VMs can communicate directly to each other. To access these VMs over the internet, a Cloud Service Endpoint is used. Each Cloud Service has a internet addressable virtual IP address assigned. And that’s the Cloud Service Endpoint. With PAT, ports for RDP or PowerShell are forwarded to the VMs by default. If you deploy a webserver and an application server, both can be provisioned to the same Cloud Service and therefore, they share the same Cloud Service Endpoint. But you can only forward http traffic to the webserver. Therefore only the webserver is available over the internet, not the application server.