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I am in the very early stages in planning a program for monitoring a VPN connection and am currently catching all ipv4 addresses from the adapters. When I am connected to my VPN it is always starting with 10 - so something like: 10.8.1.202 and every VPN I have ever used has always used this.

What I am thinking of doing is getting a list of all the ipv4 addresses and then checking to see if any begin with ( 10 ) then mark it as a possible VPN to monitor.

Do ALL Virtual Private Networks use this range? or are there others I should be aware of?

Thank you.

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2 Answers

up vote 19 down vote accepted

No. There are multiple subnets set aside for private networks:

10.0.0.0    – 10.255.255.255
172.16.0.0  – 172.31.255.255
192.168.0.0 – 192.168.255.255

See RFC1918 for full details.

People are free to choose whichever ones they want from that list. But there's no way to tell if the network is a VPN or not unless you actually know the network. By their very nature, VPNs just appear to be another link in a network.

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Thank you very much for your reply, I really appreciate it. I will now have to completely re-think my strategy, perhaps just let the user define what address will be monitored. – Shambhala Mar 8 '12 at 20:21
up vote 6 down vote

As Mark said, there are three address ranges that are reserved for private networks, and all of these can be used by/for VPNs; indeed, this can become quite troubling when you manage multiple VPN links with mutiple partners/customers/vendors, because their IP addressing schemes can overlap, making your routing a real pain.

Also, a VPN link can (although this is less common) connect your network to a publicly-addressed remote one, if someone there decided their public IP addresses can only be accessed via a VPN connection and not via their main firewall. This may sound crazy, but I've seen it actually happen.

To sum it up, there really is no way to say "this IP address is a VPN one" without detailed knowledge of how the network is layed out.

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Having public IPs hidden behind a VPN gateway is very common at organizations that traditionally have a large public address space to use: Universities. – SvW Mar 8 '12 at 20:14
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or corps that have acquired and merged many early entities, eg HP/compaq/Dec seem to have a bundle. – Tom H Mar 8 '12 at 20:31
@Massimo - Thank you for your answer. In reference to your last paragraph, I will have no intimate knowledge of the network so guessimating like I planned will have to be scrapped. I will leave it to the User to define. – Shambhala Mar 8 '12 at 20:34

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