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I have a VDSL modem that is in bridged mode. So there is basically no "local network". The NAT is of course disabled and so on since its not in routing mode. All machines in the network get WAN ip directly given to them by the modem. Basically the modem assigns external IP for each machine and theres no local network.

I have nginx server running on one machine and if I download a file from that machine by using my other machine it comes almost with the speed of 1Gbps. What I dont understand is how is everything transferred so fast since I connect to the nginx server machine with WAN ip and theres no "local area network" basically. Is there some automatic detection in the modem that knows which machines are connected and it will transfer all the information between them locally if it sees any machine connecting to some other machine in this network.

My connection speed is 100/30 Mbits and it wasnt like this before. When I used to transfer files they would be capping at 30 Mbits (which is my upload speed) but now everything I do even when using the WAN ip happens with 1Gbps speed.

Is there anyone who could explain why this happens and how is it all detected automatically, both when using IPv4 and IPv6 addresses when transferring files. Why are the speeds so high when I have no local network? Is this normal behaviour from a modem?

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You could try a tracert <wan_ip_of_other_machine> on a machine. You'll see over what physical hops/routers/modems the packets go. I expect the modem is routing them directly back to the other machine because it knows its IP address and if your local connection to the modem (link-speed) is 1Gbps it doesn't leave your location over the internet. – Rik Jan 8 '14 at 8:38
If there are two machines that are inside the same subnet and connected to a device that can act as a switch, there's a local network. – David Schwartz Apr 4 at 9:21

You do have a local network. If your VDSL modem has multiple gigabit Ethernet LAN ports, then it is effectively a small gigabit Ethernet switch.

If your machines are given IP addresses with a subnet mask that indicates they are all on the same subnet, then they will try to talk to each other directly over Ethernet rather than trying to route their traffic via some gateway on the other end of the VDSL line.

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