Carrier-grade NAT places address translation inside an internet provider’s network. Multiple customers can share public IPv4 address space. This is different from only having a private address behind your own home router, and it can matter when an application needs an incoming connection.
A practical example
If you forward a port on a home router but an external connection still cannot reach the device, a second translation layer is one possibility. It is not the only explanation: firewall rules, the application and the test itself also matter.
What to check
Ask the provider whether your service uses shared addressing and what options it offers for the task you need. RFC 6598 reserves 100.64.0.0/10 as shared address space, but an address observation alone should not replace confirmation of the provider’s setup. Explain whether you are hosting a service, joining a game or simply browsing.
What it does not tell you
A VPN subscription does not automatically provide an incoming public address or port forwarding. Check that exact feature before choosing a service. Zenshield does not claim a verified port-forwarding capability here.
Read the technical reference for the underlying definition. For a connection that is failing now, use the troubleshooting guide. To choose an app, follow the VPN selection worksheet.