Another hurdle to consider is the fact that there are technical differences between the Skype application, the online client and Skype for Business. Users on the Skype network are acting as both users and servers in their distributed methodology. Skype does not use a traditional client/server topology.

to the UTMs SOCKS5 proxy, and specify the UTM proxy in Skypes config. The protocol is proprietary but since it works on the same port as normal SSL/HTTPS traffic, it renders traditional port blocking filters ineffective. Is any one know how to block all users use Skype except some specific users. Skype's initial outbound connection can use any dynamic combination of TCP and UDP ports, including outbound ports 80 and 443. They use a very large range of IP addresses as well as a revolving list of domains in order to connect to their main service, which are all subject to change without notice.

Skype uses an aggressive and adaptive networking technology specifically designed to reach the internet. Due to the nature of the application, Skype services can not be prevented from working simply by blocking certain domains.

How to block on skype