VPN PAT refers to using Port Address Translation (PAT) in conjunction with a Virtual Private Network (VPN). Specifically, PAT allows multiple devices on a private network behind a VPN connection to share a single public IP address provided by the VPN server, but assigns each device a different port number. This is often done to conserve public IP addresses. However, it's crucial to note that Azure VPN gateway does not support PAT.
Here's a more detailed breakdown:
How VPN PAT Works:
When devices on a private network behind a VPN want to communicate with the internet, their traffic first goes through the VPN tunnel. The device's private IP address and port number are translated (using PAT) to the VPN server's public IP address, but a unique port number is assigned for each internal device.
Why Use VPN PAT?
The primary reason for using VPN PAT is IP address conservation. In situations where public IPv4 addresses are limited, PAT allows numerous internal devices to share a single public IP address, making it a practical solution for many organizations.
Example Scenario:
Imagine a small office with several computers connected to a router running a VPN client. Without PAT, each computer would ideally require its own public IP address. With PAT, all computers can share the VPN server's public IP address, but the router assigns each computer's traffic a different port number. When a response comes back to the VPN server, the port number indicates which internal computer should receive the data.
Key Considerations:
- Azure VPN Gateway Incompatibility: As stated earlier, the Azure VPN Gateway does not support PAT. If you're planning to connect an Azure virtual network to an on-premises network, this limitation is important.
- Port Availability: The effectiveness of PAT depends on the availability of port numbers.
- Potential for Issues: Some applications and protocols may not function correctly with PAT, particularly those that rely on consistent IP addresses or have strict port requirements.
Alternatives for Azure VPN:
If you need similar functionality to PAT with Azure VPN, consider these alternatives:
- Multiple Public IP Addresses: Provision multiple public IP addresses for your Azure VPN Gateway if you have a limited number of internal IPs that need to connect.
- User Defined Routes (UDRs): Use UDRs and a Network Virtual Appliance (NVA) to create a more customized routing solution, potentially handling NAT capabilities outside the VPN gateway.
In summary, VPN PAT allows multiple devices to share a single public IP address via port translation, but Azure VPN gateway does not support it. Consider alternatives if you require this functionality within Azure.
