While testing the new designs of some of our web sites, we discovered that parts of the sites (mostly banners) were not showing on our Dell desktop computer. We found out that this is due to the ad blocking features in Norton Internet Security.

After testing a little bit more, we noticed that the ad blocking feature is using various parameters to decide which parts of the page to censor. As a result, we had to rename the files of the banner rotation system to exclude any wording like “banner”, “click”, “view”, etc.

It also helps, if images are larger than standard banners. Norton Internet Security seems to block anything that uses standard banner sizes, such as 468×60 pixels, but also files named “banner.gif|.jpg”.

A brief 2 day test on one of our sites revealed that around 4.2% of browsers block files named “banner.gif”.

14923 hits for /images/about.gif (120×10 pixel)
14291 hits for /images/banner.gif (120×10 pixel)

As these files don’t use any standard banner size, we can conclude that blocking is purely done on file naming conventions.