hese companies did not need the complex information infrastructures of the earlier on-line providers, because they offered the Internet's wealth of information instead. The invention of Mosaic, the first graphical World Wide Web browser, in 1993 essentially made the earlier information services obsolete. Retail ISPs instead offered dial-up modem access, mail servers, and, in most cases, Web servers for subscribers to put up their own pages. Most retail ISPs, unlike the national on-line service providers, initially owned their own modems. But operating modems became a specialty business of its own. AOL, for instance, was one of the largest of the on-line service pro- viders, with three million subscribers, at the beginning of 1995. At that time it was beginning to phase out its use of Telenet and Tymnet's X.25 networks and install a new "AOLnet." The latter was not internally operated by AOL; it was outsourced to Sprint, BBN, and ANS (the latter, however, owned for a time by AOL). It was originally going to use UUNET, but when Microsoft announced its own competing service, MSN, UUNET took an investment from Microsoft and became MSN's key modem provider; its role in AOLnet went to BBN instead. Various other companies also set up "rent-a-modem" services, enabling retail ISPs to have a larger local-calling footprint than they could afford to provi- sion on their own. Thus the ISP industry developed three major subindustries (see Figure 4.2): Access ISPs interfaced with local telephone companies, providing modems and later digital subscriber line (DSL) services in bulk. Vertical ISPs operated servers and dealt directly with retail customers. Vertical ISPs included both consumer-oriented retailers and commercial-oriented operators, such as Web hosting companies. Backbone ISPs provided connectivity between each other and to vertical ISPs. Some providers, of course, operated in more than one category. But spe- cialization became the norm; even companies operating in more than one subin- dustry tended to have a natural se