Canaan Partners and Masthead Venture Partners led an $11 million B round in Tremor Media, an ad network that runs video ads streaming in other videos and in banner ads as well. This brings the total raised by the New York City startup to $19.4 million, including a series A in September 2006. Canaaan and Masthead led that round as well.

Tremor’s video ad network reaches a cumulative 92 million people across more than 800 publishers. According to comScore, it is one of the top-20 ad networks by reach.

Focusing exclusively on video ads is a sure path to rapid growth, but at some point advertisers are going to want to go to one ad network to place all of their ads. Eventually, Tremor will either have to expand its offerings beyond video, or sell itself to a larger, full-service ad network.