This is very likely the easiest way to block Skype advertisements using OpenWRT. Blocking at router means you do not have to apply the block on every computer that goes out via this router.
Works for: Skype for Windows as at version 7.17
On my computer, Skype is the only application that shows ads. Do I need a stronger reason? :)
Skype advertisements are served from apps.skype.com.
By blocking IP address lookup of that host name, or substituting it with one that points to nowhere, you will see no ads. It is that simple! At least until they change the way it works. In such a case, we shall play a game of Cat and Mouse.
In OpenWRT Chaos Calmer, navigate to Network > Hostnames then enter:
Hostname: apps.skype.com
IP address: 127.0.0.1
Restart dnsmasq service.
Note: This also blocks Skype Home for which I do not use anyway.
You will still see a placeholder for ads in Skype but it will now show nothing. There are tutorials that describe how to remove the placeholder by modifying Skype configuration but I noticed them coming back after an upgrade. This does not bother me so I will leave it as-is.