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I'm trying to find out the port number of my computer to which my ISP actually connects & provides the internet service, given my ADSL modem is operating in Bridge Mode instead of PPPoE. I initially wanted to find out others users' port numbers, but I thought first I'd find out my own.

I'm on a windows machine, so I used the command netstat -a -b -n but could not figure out the port id. I suppose if there IS such a port number, there must be some sort of internal port forwarding being done. So then how do I find out my port id?

Edit: Basically my question is, How does the ISP send data packets to a computer..I ask this since I had figured out a way to impersonate someone using his MAC address and session id (my ISP is BSNL). So there could also be a way of hijacking someone's connection & breaking into his system. That depends a lot on how ISPs actually send data to different clients.

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Are you asking "Which physical port of the ISP's switch/router connects to my modem?" –  jscott Aug 30 '11 at 20:41
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closed as off topic by Jason Berg, Shane Madden, Ben Pilbrow, jscott, Iain Aug 30 '11 at 20:48

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1 Answer

up vote 4 down vote accepted

I think you are searching on the wrong OSI-Layer. When it is bridged, your PC does PPPoE, and as the name says it is over Ethernet, not over TCP/UDP. So there are no port numbers.

Just have a look at the OSI-Modell on the right side of the wikipedia article: http://en.wikipedia.org/wiki/Point-to-Point_Protocol_over_Ethernet

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Why arent there port numbers? after all, ports are logical gateways into a computer..and if there is no port number, then can you explain how do I receive data from my ISP? Assuming ports are involved, does my router forward packets to different ports of my comp, or does it forward traffic only to 1 port, & from there the packets go to different ports (for diff. applications) ?? –  Rushil Aug 30 '11 at 20:34
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you should really read some literature about network-layers if you want to understand it further. every paket can be wrapped into a packet of the layer under the current layer. so your tcp packet is put into an ip packet, which is send by your pc via PPPoE to the provider, which unpacks it from the PPPoE frame and forwards the ip-packet, again using a packet/frame of a layer under ip. –  allo Aug 30 '11 at 20:44
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