NAS IP Address

Hello, I'm using free radius server 2.1.11 on Linux Enterprise Server 6.1. OS: Linux Enterprise Server 6.1 Radius: free radius server 2.1.11 Database: Mysql The WIFI routers we're using are in diffirent private networks, behind the internet gateways. The WIFI router has private IP address, such as 192.168.1.1. For some reason, we need know which Internet IP address, the WIFI router is using to do the authention with the FreeRadius server. The FreeRadius server is on internet. As the business needs, we need save the IP addres(Internet gateway IP address) to MySql database. Can anyone advise how to do so? Thanks! Tom