We just switched our Optus Cable service across to their "speed pack" which supposedly gives us 10x the bandwidth. To do so, we needed to upgrade the modem from the old Motorola Surfboard 3100 to a Netgear CG3000 which has a built-in router and does Wifi.
Our home network is more complicated than normal but I copied across all the appropriate config, rebooted all the computers and it all just worked. Well, almost all.
For some reason, Pauline's email would not connect. Nor would Catriona's. It didn't matter how I tried, it just wouldn't work. When I tried configuring new accounts, they wouldn't work either.
I probably spent half an hour (over a few nights) ranting to the guy on support that "I do networking for a living, I can see that its your mail server not responding to POP/SMTP when I use TELNET to the appropriate servers and do the protocol by hand".
Today, sheepishly, I realise that "hmmm, the IP address we are using is a class-B, but the subnet mask we have configured is a class-C". Surely that couldn't cause this symptom, it didn't upset the old modem. Hmmm, this is a modem *and* a router. Oh dear...
So, I fixed our network config to use legitimate class C IP addresses, Pauline pushed the Get Mail button and down came her mail. Problem solved.
My only excuse is that my old router allowed me to configure a proper class-B subnet mask - the Netgear has the subnet mask locked at class C, but I didn't pay attention when I was entering the numbers. Tomorrow, when Optus Support rings me back, I have to take hat in hand and admit my error.
The moral: if you want to use anything other than the default 192.168.1.* network that your modem manufacturer, or your ISP expects you to use, make triply sure that you have configured it right. Because getting it wrong is hellishly difficult to debug, especially for the poor bastard at the other end of the phone that doesn't know how clever you think you've been.