Why is my iChat not working?

Why do Airport, iChat and VPN stop working in certain coffee shops?

• Why do Airport, iChat and VPN stop working in certain coffee shops? I've been working remotely this week for the first time, and I keep getting suddenly disconnected. Sometimes when I walk in the shop and open my Macbook, the Airport logo is greyed out completely, rather than giving me a list of available networks, and won't do anything until I reboot. And then sometimes, there's an exclamation point on it and nothing I do can make it browse for a connection. Recently also, I was on iChat and suddenly I couldn't change my status to "Available" and got "could not connect" errors. Right after than, I could not VPN into my company's intranet until I went home and used my connection there. Is it a security thing? An IP address thing?

• Answer:

  Many coffee shops use a technology called Network Address Translation (NAT). It's a fancy name for sharing one internet address among multiple machines. Problem is, not all applications work well with NAT. Web and mail traffic is OK, chat is usually fine but direct file transfers might not work. And VPN is hit or miss. I have personal experience with Microsoft's PPTP having trouble. There are ways to make these work, but it depends on the router being configured properly for VPN. Most of these places don't have the money to hire professionals to setup their networks. Your average barista's idea of network troubleshooting is to reset the router (not that I blame them, it's not really their job to know). So you takes your chances. It's been a bit, but I recall Starbucks as a place that usually has a professional quality network. Of course, you pay for it (or you used to). Don't even get me started on 265 host subnets and 12hr DHCP lease times.