Skype hosts list
So, in many circumstances where UTM-device are not available, the best approach is to deny all traffic to Skype hosts. So, what are the IP addresses used to authenticate? The hosts I found are included in the following networks:
Please note that, while I masked all network with a C subnet (/24), the real subnet mask can be quite different (ie: it can be a smaller /28 netmask). However, any false positive should be easily detectable by simply enabling connections logging on your firewall.
For a complete connections log of my Skype login attempt, you can download this file.
A blocked Skype login attempt
Sure, a new Skype version and/or upgrade can boast a new hosts list, but this approach should work in a number of cases: I tested both Skype beta 2.2 for Linux and Skype 5.9 for Windows XP, and both were blocked by my gateway firewall.