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I am unable to reach my ISA Server from outside the router because the router is blocking my connect

I have a Microsoft ISA Server located behind a Cisco 800 router. The router has a public IP address and the ISA Server has a private address. I am unable to reach my ISA Server from outside the router because the router is blocking my connections. What do I need to do to reach the ISA Server?

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Because your router's external interface has a public IP address and its internal interface has a private IP address, the problem you are experiencing is probably caused by Network Address Translation (NAT) on the router. Incoming connections from the Internet to your ISA server cannot reach your ISA server unless a static NAT mapping makes the ISA server reachable through the router's public IP address.

You did not say which Cisco 800 model you are using, but there's a good NAT configuration tip for reaching internal Web servers through Cisco 827 routers posted on Cisco's website. This tip describes configuration port 80 configuration for a web server, but a similar configuration can be used to forward traffic to other internal servers using other ports. More information on configuring NAT on Cisco 800 series routers in general can be found here.

This was first published in July 2004

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