Unable to connect VPN due to double NAT issue

A double NAT (Network address translation), where internet data goes through the NAT process twice, can cause communication issues between Privacy VPN and servers. 

Problem

You're unable to connect to a VPN server.

Symptom

 The error message below displays:

MB404104: Unable to connect to VPN server. Check your network connection and try again.

Cause

Internet service providers (ISPs) provide a modem or gateway, a modem and router combo, that is set up to perform NAT. A double NAT happens when another router is connected to the ISP modem or gateway. Having a double NAT, where internet data goes through the NAT process twice, impacts VPN functionality, online gaming, and communication between devices connected to your network.

Check if you have double NAT

Windows

1. On a Windows device connected to the Internet, click the Start menu.
2. Type cmd, then hit Enter/Return.
3. Type tracert 8.8.8.8. This performs a traceroute to Google’s DNS server.
4. Compare the two first IP addresses listed to the private IP address ranges listed below.

If the two first IP addresses listed are private, you have double NAT.

Mac

1. Launch Network Utility, on a Mac device connected to the Internet.
2. Click Traceroute.
3. Enter Malwarebytes.com
4. Click Trace. 
5. Compare the two first IP addresses listed to the private IP address ranges listed below.

If the two first IP addresses listed are private, you have double NAT.

Private IP addresses ranges

Private IP addresses are within one of the following ranges:

• IPv4 private address ranges (RFC 1918):

  10.0.0.0 – 10.255.255.255 (24-bit block)
  172.16.0.0 – 172.31.255.255 (20-bit block)
  192.168.0.0 – 192.168.255.255 (16-bit block)

• IPv6 private address ranges:

  fc00::/7 address block = RFC 4193 Unique Local Addresses (ULA)
  fec0::/10 address block = deprecated (RFC 3879)

Resolution

If you determine you have double NAT, contact your Internet Service Provider for help.