This week I’m in Seattle, WA teaching Adobe Flex at the client site. Everyone in the classroom was given a password to the local Wi-Fi router. Everyone but one person successfully connected to the Internet. This unlucky guy was me.
In fact, my notebook was connected to the router, but that was as far as I could reach. OK, I was the only person using MacBook. Can this be a problem? Checked the network settings – everything looked hunky dory. I got a valid IP address. One of the students went to see Da Man – sysadmin, who gladly confirmed that my IP address has been blocked by the router’s software. Why? Yakov’s machine makes lots of connection requests to random IP addresses in a short period of time.
Students started to make fun of me – a minute ago I was explaining how to process financial data feeds from unoccupied Wall Street and students suggested that I must have been involved in a heavy-volume stock trading while teaching the class. I wish! Since May I’m working for another client sitting on a trader’s floor, where I had to sign an agreement that made trading impossible (insider’s information, you know).
Opening Activity Monitor on my machine didn’t show anything suspicious – the only running programs were MS PowerPoint, Acrobat Reader, Eclipse, and Skype. Another student suggested that it might be some virus or antivirus software installed on my machine. Wrong. Ain’t using Windows – me no need antivirus. Me no have viruses.
How to find out who makes all these network calls? Good old Charles network monitor came quite handy, as usual. I started this sniffer, and sure enough, every couple of seconds a connection attempt was being made to some IP addresses…
After shutting down Skype, the problem was solved. Skype was the bad guy trying to make all these connections! This makes me wondering, does Skype tries to poll each of my contacts who’s online? This must be the case. We reported my findings to the system admin, who unblock my IP, and the next second I saw Google!
The lesson learned: always have an HTTP sniffer with you if you ever want to see the light of Google’s home page.
yakovfain.com 
Skype is a peer-to-peer service that uses a distributed network of “supernodes” to facilitate communication. So you share your computer power and your Internet connection. Also for a longer battery life, turn off Skype when not needed.
Download a copy of Little Snitch (http://www.obdev.at/products/littlesnitch/index.html) – comes in useful for allowing per-application firewall rules as well as monitoring detailed network usage.
There are problems with Little Snitch under Lion. Make sure you know what you’re installing, read the forum there.