Hi Prathish,
As Sandip has suggested, you can consult your network team to set up a reverse proxy in DMZ so that your ESS / MSS apps can be accessed over Internet.
You can also look for SAP Web Dispatcher and can configure it to act as a reverse proxy.
In our landscape, we've used SAP Web Dispatcher that is kept in DMZ and accessible from outside. This web dispatcher makes 302 redirect calls to our portal server and R/3 back end server to access WDA apps for ESS / MSS.
Please let me know should you need any additional info on how to configure SAP Web Dispatcher for accessing the SAP back end apps through Portal on Internet.
BR,
Anurag