The Internet Before Commercialization
Early history: the development of separate networks for military research (DARPANET) and non-military research (NSFNET) were merged, and network access granted to Universities. These were old-school systems, mainframe and dumb terminal.
This coincided with an explosion in personal computing, replacing the dumb terminal with a "personal computing" device that had processing power, such that the central computer could be relieved of most data-processing functionality and become a repository of data files that could be accessed by remote computers.
Meanwhile in the private sector, dail-up services such as AOL, CompuServe, and Prodigy were providing users access to central data systems through direct dial-up connections through a network of modems.
The internet emerged when DARPANET and NSFNET were interconnected. This was done by creating standard protocols (history: a "protocol" was originally a leaf glued to a scroll that indentified the nature of its content) for data interchange.
A few other networks that were in existence previously (notably, CSNET, BITNET, and a few state university systems' private networks) adopted the standard protocols and merged into the Internet. The internet also went global, with overseas networks (CERN in Europe, notably) linking up.
Commercial networks were permitted to connect, but there was a strict "acceptable use policy" that limited their ability to communicate information. Specifically, "for-profit activities" were expressly banned, as was "excessive use for private or personal business"
In 1989, a regional network (NYSERNET) and a private for-profit company (PSINET) began selling network access to other commercial customers. This caused quite a stir, but the letter of the policies did not prohibit the sale of access (only its use for commercial purposes).
Also worth mentioning (though the author skips it) is the numerous privately-operated "bulletin board systems" that were run by private citizens (computer hobbyists) to looked to the Internet as a better method of making their resources accessible to their users, who previously had to take turns dialing in via modem to a single computer.
The Science and Technology Act of 1992 inadvertently opened the network up to commerce:
[The NSF] is authorized to foster and support access by the research and education communities to computer networks which may be used substantially for purposes in addition to research and education in the sciences and engineering, if the additional uses will tend to increase the overall capabilities of the networks to support such research and education activities.
The vagueness of the underscored passages was interpreted to mean that the network could legally be used for purposes other than research and education.
A side note on Al Gore: he once stated that "I took the initiative in creating the Internet," which was widely taken out of context to suggest that he claimed credit for its creation. He chaired a committee that created the High Performance Computing Act of 1991, which proposed increasing the backbone capacity of the NSFNET, but wasn't involved in the original creation of the network, nor the 1192 act opening it up for purposes other than communication among research centers.
There's some discussion about the emergence of top-level domains (com, org, net, mil, and the country-codes), the way domain names were created and assigned, etc., but this is largely just trivia. Its' worth mentioning that the NSI, which held a monopoly on domain name management for a long while, was a spin-off of the GSI, split out as a private corporation to appease international insistence that the US government's control over their networks. Much later, control was sifted to an international organization (ICANN) who handled management tasks, but was not in the business of selling domain names to individual customers.
And so, before the Internet "went commercial" in 1993, much of the infrastructure was already in place to support the needs of private enterprise.