How to get client's default gateway?

Hi! Can you help me on this matter?
I need to determine the default gateway of client's pc accessing on the server so that I can know their location, I saved every client's default gateway but how can I know it when they login?
I used this code: 
  For Each networkCard As NetworkInterface In NetworkInterface.GetAllNetworkInterfaces()
           For Each gatewayAddr As GatewayIPAddressInformation In networkCard.GetIPProperties().GatewayAddresses
             clientGateway = gatewayAddr.Address.ToString()
          Next
        Next
But when they login the default gateway determined was the server default gateway..

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