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Certpedia: Terms That Start With C

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  • CAPTCHACAPTCHA - Developed by Carnegie Mellon University, CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart". This system was developed to prevent internet bots used by spammers to automatically fill out forms on web sites and submit them. On a form, CAPTCHA displays a series of distorted letters and numbers that must be correctly entered in order to submit the form. Recently, spambots have begun using character recognition software that is able to bypass this in many cases.

  • CBT (Computer Based Training) - Traditionally, computer based training is composed of self-paced tutorials, tests, and simulations that a student can study on their computer. In the certification industry, most CBT products consist of classroom-style videos sold on DVDs. The benefits of CBTs over classroom training is the much lower price, the ability to take your study materials anywhere, and the ability to skip around or repeat portions of the video as necessary.

  • CGI (Common Gateway Interface) - A Common Gateway Interface (CGI) is a standard protocol for interfacing external application software with a web server. This allows the server to pass requests from a client web browser to the external application. The web server can then return the output from the application to the web browser. On the web server, there will usually be a cgibin directory where PERL and other CGI applications are stored.

  • CHAP (Challenge Handshake Authentication Protocol) - A type of authentication protocol used on PPP connections. CHAP uses a 3-way handshake in which the authentication agent sends the client program a key to be used to encrypt the user name and password. CHAP not only requires the client to authenticate itself at startup time, but sends challenges at regular intervals to make sure the client hasn't been replaced by an intruder, for instance by switching phone lines. For more detailed information, click here.

  • CIDR (Classless Inter-Domain Routing) - Classless Interdomain Routing (CIDR) was introduced to improve both address space utilization and routing scalability in the Internet. It was needed because of the rapid growth of the Internet and growth of the IP routing tables held in the Internet routers. CIDR moves way from the traditional IP classes (Class A, Class B, Class C, and so on). In CIDR, an IP network is represented by a prefix, which is an IP address and some indication of the length of the mask. Length means the number of left-most contiguous mask bits that are set to one. So network can be represented as CIDR also depicts a more hierarchical Internet architecture, where each domain takes its IP addresses from a higher level. This allows for the summarization of the domains to be done at the higher level. For example, if an ISP owns network, then the ISP can offer,,and so on to customers. Yet, when advertising to other providers, the ISP only needs to advertise

  • Circuit Switching - In circuit switching, a dedicated path is built between the source and destination, and the packets will all be sent over this dedicated path. If this sounds like a telephone call, you're right! A phone call is a great example of circuit switching. On a Cisco router, ISDN is a circuit-switching technology. Introduction to WAN Technologies

  • Cisco Networking Academy - The Cisco Networking Academy Program is a comprehensive e-learning program that provides students with the Internet technology skills essential in a global economy. The Networking Academy delivers web-based content, online assessment, student performance tracking, hands-on labs, instructor training and support, and preparation for industry standard certifications. Launched in October 1997, the Networking Academy has spread to more than 150 countries. Since its inception, over 1.6 Million students have enrolled at more than 10,000 Academies located in high schools, technical schools, colleges, universities, and community-based organizations. Cisco Networking Academy Program

  • Clustering - Clustering is the use of multiple computers and storage devices that are virtually combined together to create what seems to be a single system. Clustering is often used to increase a system's availability, performance, and for load balancing on highly-trafficked Web sites or database servers.

  • Coaxial CableCoaxial - This cable, often referred to as "coax", is mainly known for its use with cable television systems. It was once used on early Ethernet bus networks (10Base-2), however, it has been replaced for this application by twisted pair and fiber optic cabling. More recently, this cable has proven itself useful for broadband internet connections because of its ability to transmit large amounts of data over long distances. This cable is made of a central conductive core that is wrapped in an insulator and then a second conductive layer. These three layers are then typically wrapped in an outer insulating layer.

  • CompactFlash - CompactFlash is the oldest and largest of the flash card types and is based on a simplified PCMCIA bus. CF cards come in CF I and CF II sizes, the latter being thicker. Like all other flash cards, the CompactFlash cards are solid state, although some manufacturers have turned this form factor into microdrives which are minature hard drives. These drives have platters and heads just like a regular hard drive. The two types look identical, however, microdrives can use too much power for some devices, but are less expensive.

  • Convergence – Convergence is achieved when all of the available topology information from routing devices have been passed along to all of the other deceives in totality and all when the information gathered is not in a contradiction state to any other router's informed topology information. When all of the network routing devices "agree" on what the network topology looks like it is said to have full convergence.

  • Cookie - Cookies are unencrypted text files stored on the client computer that store information about the user for use on a web site. They are commonly used to customize pages for you based on your browser type, or other information and preferences that you may have provided the Web site. Most browsers will accept cookies by default, however, the security settings can be changed to reject them. If this is done, some web pages will lose functionality or not work correctly. An expiration date can be set in a cookie. If no expiration is set, the cookie will expire when the browser is closed. A single domain is restricted to only setting 20 cookies and each cookie can be a maximum of 4KB in order to prevent client machines from being inundated with cookies and filling up hard drive space.

  • CPUCPU (Central Processing Unit) - A CPU, also known as a processor or microprocessor, can be thought of as the brains of a computer system and is responsible for executing software commands and performing calculation functions. Microprocessors

  • Cryptography - Cryptography is typically concerned with the processes of scrambling ordinary text (known as plain text or clear text) into encrypted text (Ciphertext) at the sender’s end of a connection, and decrypting the encrypted text back into clear text at the receiver’s end. This is accomplished using a specific algorithm and a secret key. It is used to protect e-mail messages, credit card information, and other forms of electronic data. Common types of cryptography include Public-key Cryptography and Symmetric-key Cryptography.

  • CSMA/CD (Carrier Sense Multiple Access with Collision Detection - In the early days of ethernet, when two hosts would send packets at the same time, a collision would occur. A standard had to be created that would have the hosts follow rules relating to when they could send data and when they could not. This standard is Carrier Sense Multiple Access with Collision Detection, referred to as CSMA/CD. CSMA/CD forces computers to “listen” to the wire before sending in order to make sure that no other host on the wire is sending. If a collision is detected, both of the senders will send a jam signal over the Ethernet. This jam signal indicates to all other devices on the Ethernet segment that there has been a collision, and they should not send data onto the wire.
    How Ethernet CSMA/CD Works

  • CSS (Cascading Style Sheets) - CSS provides an easier solution to site updates by storing formatting paramaters for all of the text, layout, and other elements in a single master file. The .css file is denoted in the HEAD section of the HTML document and individual settings are called using the CLASS attribute with HTML tags. CSS Tutorials

  • CSU/DSU (Channel Service Unit/Data Service Unit) - This is a piece of equipment that connects a leased line from the telephone company to the customer's equipment (such as a router). Although CSU/DSU's look similar to modems, they are not modems, and they don't modulate or demodulate between analog and digital. All they really do is interface between a 56K, T1, or T3 line and serial interface (typically a V.35 connector) that connects to the router. Many newer routers have 56K or T1 CSU/DSUs built into them.