paration between the lines of business. Backbone providers sold directly to large businesses and to downstream vertical ISPs, a business-to-business sales model built on large sales departments and small billing departments. Vertical providers selling to consumers had a mass- market sales model, typically billing via credit card. Few companies could make both sales models work together, SO even those who began trying to be full- service providers often abandoned some markets. Web hosting was a form of vertical ISP business that evolved naturally out of backbone provision. Before the World Wide Web became such a dominant application, companies with Internet connections generally maintained all of The Internet Boom and the Limits to Growth Long-distance telecom carrier Large business customer relationship Rent-a- Internet backbone Upstream Internet vertical modem Internet access service provider service provider service provider Peering Retail Telephone customer lines (PRI) relationship Internet backbone service provider Local telephone company Figure 4.2 Internet service provider industry model. Arrows indicate cash flow. their own computers on site. And the first Web servers were generally at their companies' sites, just another machine in the data center. But as Web band- width grew, the cost of local bandwidth, from the ISP backbone to the cus- tomer, became greater. Because public Web servers had little community of interest with their owners' own internal networks, it made sense to bring them to the backbone, rather than bring the backbone to them. This led to the growth of the Web hosting industry, led by the backbone ISPs. So, for instance, in 1995, BBN Planet introduced its Web Advantage service, and located several racks of Web servers in a large computer room that had once housed the parent company's DEC-10 mainframes. As the service grew, more of the company's internal computers were pushed aside to make room for Web servers, and more "Web farms" were built in California and elsewhere, at sites where th