How do I... Provide remote assistance on computers behind a NAT device?
Source: Brien Posey
Although Microsoft Windows doesn't natively support NAT traversal for remote assistance sessions, there is a way that you can provide remote assistance to someone who is behind a NAT firewall. However, doing so requires a little bit of tweaking by your user. This How do I.. shows how to use port forwarding to get around a NAT firewall when using XP's remote assistance feature.
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