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I have a switch connecting several computers with my ISP.
All the computers gets external IPs from my ISP. When tracing the route between two of the computers the traffic always passes through my ISP before returning to the correct computer (which reduces the transfer speeds between computers to my internet connections speed).
I was under the impression that the switch should route traffic directly to the correct computer.

What am I missing, and how can I accomplish this?

I also have a router/AP that can be used, but it doesn't have DMZ.

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up vote 3 down vote accepted

The complication you are seeing here is one of switching, subnets, gateways, and routing.

However it does not strictly explain what you are seeing here.

What should be occurring is that traffic for an IP address that falls outside the range specified on the interface will be sent to your default gateway to leave your network. Reading into your setup there, this will be the one that your ISP provides at the other end of your connection (as opposed to a local one forming a border for your local traffic).

A router sitting on the end of your connection makes sense - if only for accounting and security purposes.... however it does not make clear, if the switch is capable, and the subnet is correct, then why an ARP request does not result in an IP and MAC match within the subnet and another port on the switch; resulting in the switch simply pushing it out to the box in question?

Do you have any ARP and traceroute data to hand or any configuration from the switch?

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Switches work at the Data Link layer. IP operates at the Network layer. Since the switch cannot read the eventual destination of the packet, it must blindly follow the destination given in the datalink frame, which is the ISP. The ISP's machine then reads the network frame, and sends it back into your network with the eventual destination now in the datalink layer.

In order to prevent this you need a machine between your LAN and the ISP that can read the network frame and redirect it appropriately. Unfortunately most SOHO routers are not designed to operate in the manner you need, and you will need an actual computer to act as a DHCP relay to the ISP while being able to route packets appropriately.

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Uhm. Assuming all PCs of the OP have a IP in the same subnet then they can talk dirextly with eachother and a switch should do so once it learns the relevant ARP addresses. The OP could easy take any cheap SOHO switch and gain that functionality. – Hennes Dec 10 '15 at 19:09

A switch is typically used to provide communication between devices on a network. A router is typically used to connect networks.

In your situation I'd place the router between the switch and the internet. Most home routers support DHCP, so the router will become the DHCP server for your LAN and will receive a single IP address from your ISP. This way, all communication between devices on your LAN will not be routed to the Internet, but genuine Internet requests will.

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