The Scenario

A remote user connects to the VPN and the client status shows “Connected.” Curiously, while the user can successfully ping an internal file server at 192.168.1.50, they are unable to map network drives, DNS resolution fails, and internal web applications refuse to load.

In an MSP environment, this “Ping Paradox” often leads Tier 1 technicians to believe the tunnel is healthy. However, as an escalation engineer, I recognize these as the classic symptoms of an IP Subnet Collision.

The Technical Deep-Dive

A collision occurs when the user’s Local LAN (Home Wi-Fi) uses the exact same IP address range as the Remote Corporate Network. Because the vast majority of consumer routers default to 192.168.1.x/24 or 192.168.0.x/24, routing conflicts are inevitable when corporate networks utilize these same common ranges.

The Mechanics of the “Ping Paradox”

The ability to ping despite a collision usually stems from two networking behaviors:

  • Longest Prefix Match: If the VPN client injects a specific host route (192.168.1.50/32) into the routing table, that route is “more specific” than the local LAN’s /24 route. The ping succeeds because it follows the narrowest path, even though broader traffic (like DNS) is still being routed to the local home network.
  • Interface Metric Priority: If the VPN’s virtual adapter is assigned a lower “Metric” (higher priority) than the local Wi-Fi, the OS will attempt to route ICMP traffic through the tunnel first. Higher-level protocols, however, often fail to initialize when a conflicting gateway exists on two active interfaces.

Engineering Solutions

  • Network Re-addressing: The most robust long-term fix involves moving corporate production networks to “uncommon” RFC1918 ranges (e.g., obscure blocks within 172.16.x.x or 10.x.x.x) to stay clear of consumer-grade defaults.
  • Policy NAT (Network Address Translation): When re-addressing isn’t an option, we can configure the VPN gateway to “mask” the internal network as a different range for remote users. This allows connectivity via a non-conflicting IP without requiring internal server changes.