ADSL Router IP Address Weirdness

Posted on: 01 December 2009

Bit of a weird problem this week about our customers not being able to access our pre-production websites. We have a test server in our office which we use to host websites during development so our customers can see progress and add their own content. Our internet access and networking is (mostly)  handled by a consumer-grade Netgear router/firewall/ADSL modem job, which is configured to route inbound http requests to our test server. All worked fine.

This week we got a few reports of customers not being able to access their pre-production websites. We can access those sites just fine from our side of the firewall. A quick look over the router config and the test server Apache configs showed nothing that would prevent external access to the websites, and external access was working fine a week ago.

So what had changed?

Last week we had an outage from our Internet Service Provider, Pipex. This was traced to a fault at the local phone exchange, which would take a few hours to resolve. In the meantime, I thought we could try using my phone (a 3G Samsung Pixon with all-you-can-eat data contract - thanks T-Mobile!) as a shared internet connection. I can already use the phone as a modem (one of the reasons I chose it over an iPhone), so setting up Internet Connection Sharing on our Windows network should be fairly straightforward.

One of the "quirks" (or "can't be bothered writing a configuration form") of Windows Internet Connection Sharing is that the host computer (i.e. the one with the connection) must have a static IP address of The other PCs on the network must then be configured to use this as their default gateway address. The only hitch on our network is that our Netgear router has a default IP address of

Luckily, our friends at Netgear had thought that this could potentially cause some conflicts (although admittedly unlikely in their main consumer market), so provided an option to change this. I duly changed the router's IP address, and everything was peachy.

Until our customers tried to access their pre-production websites.

Realising that the router IP address change was the only thing that has changed on our network recently, I tried changing it back, and lo and behold, external access works just fine again. I have no idea why the internal network address of our router would prevent external user requests being routed to our test server. But we have happy customers again, which is kinda important.