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Textbook Passage Marking Exercise
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Directions: Print a copy of this computer science textbook passage and read it. Then go back and mark it by (1) underlining the main idea and (2) numbering the supporting details in each paragraph. All three paragraphs have stated main idea sentences.


Organizations have to be very careful to protect their information systems. To protect their information systems, companies use a firewall, a security system designed to protect an organization’s network against external threats. It consists of hardware and software that control access to the company’s intranet or other internal networks.

Typical organizational firewalls include a special computer called a proxy server that decides whether it is safe to let a particular message or file pass into or out of the organization’s network. This computer is a gatekeeper: all communications between the company’s internal networks and the outside world must pass through it. It evaluates the source and the content of each communication. Unauthorized communications are denied access.

Of course, end users have security issues as well. They are subject to many of the same types of security concerns that face organizations. Additionally, they need to be concerned about the privacy of their personal information. Today, personal firewall products are available for home users. They are available for almost any device that connects to the Internet, and they are essential for security.

Source: Adapted from Timothy O’Leary and Linda O’Leary, Computing Essentials 2007: Introductory Edition, pp. 270, 302, 317. Copyright © 2007 McGraw-Hill/Irwin. Reprinted by permission of The McGraw-Hill Companies.

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