Re: VPN through a router that has a dynamic IP; problem?

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From: Robin Walker [MVP] (rdhw_at_cam.ac.uk)
Date: 02/03/05 

Date: Thu, 3 Feb 2005 14:28:13 -0000

Charles Robertson <CharlesRobertson@discussions.microsoft.com> wrote:

> That is very useful information; I will change the relevant IPs and
> subnet, so that they do not clash and then send you the results...
>
> However, I do not understand why this is the case?
>
> Surely if the IP I access the server with is an external IP (public),
> the server's computers will never see my LAN IP; I do not understand
> why there should be a clash...

You said you were setting up a VPN. The effect of the VPN is to set up a
virtual link directly between your LAN addresses and the LAN addresses
behind the office's router: both ends are ignorant of the public IPs of the
router(s) by which you create the VPN connection.

For IP routing to work properly on the virtual link, the IP sub-net at the
far end must be distinct from the IP sub-net at the local end. Once sub-net
is accessed by broadcasting ARPs on the local LAN, the other sub-net is
routed via the VPN gateway. 

-- 
Robin Walker [MVP Networking]
rdhw@cam.ac.uk

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