JAMF resources link has a great overview of Making Apple Push Notification Service Available On Your Network.
The Apple Push Notification Service (APNs) forwards notifications between MDM solutions to Apple iOS and OS X devices. The device makes an accredited and encrypted IP connection with the APNs, receiving notifications over this persistent connection via APNs. APNs is a critical part of configuring and securing mobile devices. If there are things that prevent the direct and persistent connection to and from APNs then the entire MDM management capabilities will be inoperative.
Link: https://www.jamf.com/resources/making-apple-push-notification-service-available-on-your-network/