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We have some devices connected together through a switch. We need to access them through another network, but because of limitations in available IP addresses, we can't get an IP address from that network.

Q: How to access my devices through an external network? What kind of device should be used?

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Edit:
As routers are suggested, I have made some search and found that there is two scenario: sending from local to remote, sending from remote to local, the later is more difficult to me because the remote have only router ip as its dest address, and to send to the right local computer, the only choice is Port Forwarding?

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    You'll need a router between your switch and the 'other network'. – Zac67 May 22 '18 at 8:16
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    Routers route packets between networks. – Ron Maupin May 22 '18 at 10:29
  • I've edited my question, Is there any choice other than port forwarding when remote computer initiate a connection? – SAMPro May 23 '18 at 5:12
  • Port forwarding is only for NAT (the NAPT version). Routing doesn't require port forwarding. You forward ports for NAPT, and you only use that if you must. – Ron Maupin May 23 '18 at 5:14
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    Then you are trying to run before you can walk. There is a huge amount of stuff to learn before you get to routing, and then routing will make sense. After you learn routing, then you can get into hacks like NAPT. – Ron Maupin May 23 '18 at 5:26
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You may work with router, but security threat always be there. So, firewall is the best option logically as well as security purposes.

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  • How can I connect directly to one of my device through a router? I know the router itself has an IP address and the address of my device totally different. Sending packet from local net to outside is easy, router save the packet owner address in its table , but the opposite? – SAMPro May 22 '18 at 9:38
  • @SAMPro, routers will have an IP address on each router interface, and routers route packets between networks, so each router address will be in a different network. A router doesn't care in which direction the traffic is traveling, only that it knows how to reach the destination network. – Ron Maupin May 22 '18 at 10:43
  • @RonMaupin, what is challenge for me is connecting the remote computer to a local computer. Please see my edits. – SAMPro May 23 '18 at 5:14
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You can use firewall or if your current switch has is NAT feature For local to remote traffic :use source NAT It will NAT with PAT, all your internal system IP into ONE EXTERNAL ip. Remote to local traffic: Destination NAT one to one, it will nat any external traffic to your local system IP.

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You can do it with a router or a firewall. You have two scenarios:

Access from local to remote: This will be simple. Just use the router or firewall's LAN IP as the default gateway of your devices. notice that the LAN IP of router and your devices must be in one subnet.

Access from remote to local: You have one single public IP. After setting the default gateway you can configure port forwarding. for example if you want to access windows remote desktop, you must forward external port number 3389 of the public IP to the port number 3389 of your windows machine's local IP address. Thus, You can use just one public IP to access many services in multiple devices.

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The proper way here would be either a GRE or a VPN tunnel between the two sites, both of which do require a router on the remote location (and of course on the local). But you need to have a router already there, assuming those devices have access. The original question / diagram unfortunately does not provide enough info to suggest a proper solution (for instance: do these devices have access to internet now? Is that (?) part of the diagram actually internet or part of some partner's network? Are there private or public IP addresses on the remote devices? Is the switch maybe a layer 3 device and supports some limited routing functions?) As it stands now, those devices cannot access internet unless there is a router somewhere, or that switch provides routing and NAT functions to the local network.

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