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I encountered strange setup on a network at company where I do some work. They have internet connection from small ISP located in the same building, all I see is one CAT5 cable with 2 RJ45 connectors. One is going to WAN port on the router and another into LAN port on the same router. This is the only way for internet connection to work. I tried to connect my laptop to one of the RJ45 but it won't work.

Now, what is the purpose for such setup?

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What is the router make & model? –  Bryan Mar 19 '10 at 23:55
How is the internal network connected to the internet? –  tomjedrz Mar 19 '10 at 23:58
1) Some TP Link generic ethernet and wifi router, I don't remember exact model, but it's nothing sophisticated. 2) The rest of the network is connected to the router through switches. –  mr_stavo Mar 20 '10 at 0:02
If internet is connected to the WAN and LAN router ports, what router port is connected to the internal network? –  tomjedrz Mar 20 '10 at 0:19
As it's normal to have two plugs, one one each end, could you please clarify. Do you mean a branched cable with two plugs on one end? –  John Gardeniers Mar 20 '10 at 1:40
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2 Answers

Since ethernet requires pins 1, 2, 3, and 6, I am assuming that the other end of the cable also has two heads. They have then used 4, 5, 7, 8 for a second run using the same cable. I'm not sure what you'll find on the other end of that cable, but, that is my guess as to what they did to save themselves from running two cables.

My guess is that there is another run somewhere on the switch that the run returns to so that you have a NAT box in your premise, and it runs back to their other switch that has a VLAN that is then connected to your network.

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Thank you for the answer, so if I understand correctly my router is serving network for someone else? I need to check that, but as I wrote earlier I cannot connect to internet with only one of these plugs - I checked that with my laptop :/ –  mr_stavo Mar 20 '10 at 8:27
Possible Scenario: ISP -> your company router -> ISP vlan -> your company network My theory is that the ISP ran a connection to a router that is in your possession, backhauled that to their network and put your real connection in a port on their switch in a VLAN. You would be isolated from their network (if the VLAN was set up correctly). This would put a firewall/NAT box on your premise. The other end may connect to your network. You really need to find the other end of that cable to see what it is... and then document it. –  deleted Mar 20 '10 at 17:17
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The question is incomplete, so I reserve the right to edit when I get home tonight!

Most likely there are two separate internet connections "bonded", and that whoever connected them jury-rigged from an existing cable rather than run a second one for the second connection. Most likely the labels (LAN/WAN/AUX etc) are the standard configuration, but the ports can do other things through configuration.

A T1 connection uses either 1 pair or 2 pairs (?), and a CAT5 cable has 4 pairs. T1 is low speed for CAT5, so it shouldn't cause problems running two over a single CAT5 cable. This isn't a good practice, but if it is working I wouldn't change it.

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Hm.. I'm not sure I understand you.. If my ISP is serving two separate connections why one plug is in LAN port on my router? As for "bonding" the connection is crap (1Mbps) and is one of the reasons for my interest in that matter. –  mr_stavo Mar 20 '10 at 8:31
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