Anand I don't quite agree with you
😉 .
With Sify we all have LANs with their respective gateways. Immediately beyond the neighborhood LAN Sify's network is implemented by using what's called PMP (Point To Multipoint). See the following quote and the accompanying web-page from where I quote :-
After evaluating a host of technology options, the company opted for a wireless broadband technology called Point to Multi-Point technology (PMP), which enables customers to be linked through a wireless radio network.[/b]
--from
http://www.expresscomputeronline.com/20021209/cover.shtml
Thereafter visit this link to see a layout of PMP systems
http://rawcon.org/rawcon99/soltesz.pdf.
Thus immediately after a neighborhood LAN what we have is a Subscriber Unit (SU) which is also a Wireless Broadband
router. Here's a photo of one such SU --
http://www.axxcelera.com/online/ab_access.html
This SU wirelessly connects to a Base Station where there's the Access Point which is another Wireless Broadband router--
http://www.sifybroadband.com/ctv_operator.html
Now no authentication is done on a LAN gateway. All authentication is done on the SAM server which for each of us lies beyond the Access Point somewhere.
We seem to have different SAM servers between different cities and even within a city. But I believe there's a mechanism of sharing information between them, hence the Sify claim for us to be able to login from anywhere in India where Sify is present.
Beyond the above mentioned Access Point not only is authentication handled, but also bandwidth management on per user basis is done (giving us sh***y speeds and imposing limits on us) and external IPs are @$$igned to us (using DHCP).
So my conclusion is that all the computers on the Sify network which are behind the SAM server should be able to communicate with eachother directly without logging in. People with internal IPs 10.same.x.x usually should be behind one SAM server only and be able to connect with eachother directly. But then some cities might have started to have more than one SAM server, for eg. Delhi, and this could cause problems even when the internal IPs remain of the same form.