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Public IP Addressing

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by james_simon 1 view

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Backround:
I have a range of public ip address reserved for my company that I do not use. I am moving one of my offices into a smaller building where there is already internet access. I will be putting a firewall behind the router that is already there. There is another company that I will be sharing the office with that this is their router. I will be configuring a VPN solution to access that office from the main office and this VPN will go PTP from one firewall to the other, thus passing through the router. I need to configure my firewall with a public IP address so I can reach it via VPN.

Question: Can I assign my firewall one of those IP address that I am not using and just give their router a alias with another one of my public IP addresses? The other company only has 1 public IP address.
Thanks,
Jim

7 replies

by ins413

I may not be getting this right, but I will give it a shot. It sounds as if the only problem here will be routing. Will the router translate your internal IP addresses to your public address? Seeing as how you are sharing a router with another office, it may have to be configured to map your internal IP addresses to your external IP address, as well as the other company's internal IP address to it's external IP address. There cannot be any conflicts within offices so you will have to find out what ranges they are using internally, and consider using a different range or subnet for your internal scheme. Thern the router will have to be set up to map ONLY your subnet to your externaal IP address. There is a much simpler solution, but itwould require you to share the external IP address of the other company. Nonetheless, it comes back to router configuration. The VPN part seems to be the less complicated portion of this equation. Good luck!

by techkid

I highly doubt that it would work but, you'd have to ask that question to the other companies ISP. Your IP's were assigned through a different ISP I'm guessing. For security and for reasons to prevent people from 'leeching' inet access off the othercompanies connection, the other companies' ISP may have an ACL on the internet gateway to only accept traffic coming in from the IP the other company was assigned. That happened to me before.

You can always try it though

by justemazing

First, the ISP who assigned the public space will have it routed to your main office which means you would have to begin using at least the gateway address for the public space at your main office before anywhere else. You could then do router configs at the main office to route traffic to the new office. This whole thing is more trouble then it's worth.

Solution: If the other company is not doing any VPN stuff and has NAT running on the router, you can assign your firewall an internal address (on their subnet or create a new subnet on the router for you, if possible.) Then, configure NAT to forward the VPN ports to your internal firewall IP address. You would use the external public IP of the router when connecting via VPN and the router will send the traffic to your firewall box. This will NOT work if the other company is also utilizing VPN services with NAT turned on.

Much depends on the router at the new office and its capablilities. There are more advanced configs which will work even if the other office is using VPN with NAT, but I won't go into them here.

Another solution is to get public IP space from the other company's ISP.

--
Rob

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