Networking – How Does a VPN Manage Local IP Addresses

ipnetworkingvpn

When you have devices with the same static IP addresses on two networks how does a VPN keep them straight?

Example: I have a small network in my home office with 3 computers. I routinely connect to a remote office that has a Sonicwall router w/VPN using the Sinicwall VPN client on my main machine. Once connected, I am still able to access devices on both networks which use static IPs (they are servers.)

Both LANs use 192.168.0.x addressing. When I specify using a device like a server or printer at say 192.168.0.10 how does it determine whether to use the address on my physical local LAN or the one on the remote LAN?

Is it doing NAT translation similar to what the router does when it connects to the Internet?

Am I just "lucky" it's working?

Best Answer

To answer your question, Yes, you're "lucky" it's working.

When you connect to your VPN, your tunnel interface is assigned an IP by that remote DHCP server. So you're technically routing from the VPN server to your remote devices. You're probably connecting to your servers via their DNS names or something, which is resolved by your DNS servers locally. In which case if it couldn't find it, it'll search in your remote office.

So technically yes, this'll work. But it is not very efficient.

EDIT**

VPN diagram

So for eg. your VPN client is assigned an address by the SSL VPN. Which could be a different network address range. (eg. 172.16.32.0/16) While your remote network is working in the 192.168.1.0/24 range. The VPN server can be configured to bridge the two networks together.

The IP that your VPN client gets is not from the remote site's system/router's DHCP server, but rather the VPN server's DHCP server.

About the DNS, I don't really know how to explain properly. It uses a mix of DNS, ARP and RARP requests to determine the correct device. Hopefully this is enough to understand.

Related Solutions

Windows 7 – Fix Persistent Routes and VPN Connection Issues

Honestly, there's a few bits as to why this wouldn't work properly. But lets skip that for now.

Rather than adding an arbitrary route to an IP address, try adding the route with the "interface" specified.

i.e. when connected to the VPN if you do a:

route print

and look for the "Interface" in the list, it should have a number assigned to the VPN interface.

 19...00 0e 2e 65 ca 61 ......Realtek RTL8139/810x Family Fast Ethernet NIC
 12...00 26 55 44 95 3c ......Broadcom NetXtreme Gigabit Ethernet
 28...00 00 00 00 00 00 ......MyVPN Interface Thingie
  1...........................Software Loopback Interface 1
 13...00 00 00 00 00 00 00 e0 Microsoft ISATAP Adapter
 11...00 00 00 00 00 00 00 e0 Microsoft Teredo Tunneling Adapter

in my example... #28 is my vpn. So, when you add the route... do this:

route add -p 10.0.0.0 mask 255.255.255.0 192.168.0.1 if 28

What this should do for you is add the route... only when that interface is up. It will still sit in the "persistent routes" ... but won't actually be added to the routing table until that interface is up... and it gets removed again when the interface goes down.

Networking – Share a VPN connection with non-VPN capable devices

I guess that you are trying to share an Internet location that is available via VPN.

If WiFi is an option, than the following free products can share the connection from the VPN-connected computer :

Connectify
Virtual Router

Please note that if the VPN server dictates a compulsory tunnel, then logging-on to it will cut off any other access to any external computer, including the LAN, so no sharing is possible.

Best Answer

Related Solutions

Windows 7 – Fix Persistent Routes and VPN Connection Issues

Networking – Share a VPN connection with non-VPN capable devices

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