This paper examines theoretically and empirically the welfare effects of differential pricing for Internet traffic in a network market. We begin by analyzing a model of differential pricing by a monopolist Internet service provider (ISP), where charges are levied on content providers for traffic flow and on consumers for Internet access. Content providers differ in terms of their demand for Internet traffic and their value to consumers (“network effects”). Under linear demands, we show that compared to uniform pricing, differential pricing based solely on network effects is welfare-enhancing, while purely elasticity-based differential pricing reduces content provider surplus and social welfare. The welfare effects become ambiguous when both network effects and demand elasticities differ across content providers. Using a unique dataset on monthly transactions between a large ISP and major content providers in China (where ISPs legally own Internet traffic services in the form of CDN), we estimate the model and quantify the welfare effects using the demand and cost estimates. Our counterfactual analysis shows that consumer surplus and content provider surplus increase under differential pricing; however, a disproportionate share of the welfare gain is captured by several big content providers, while smaller content providers tend to become worse off.