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HTML

Introduction

Ithe basics of HTML—what it is, what it does, its history in brief, and what the structure of an HTML document looks like. The articles that follow this one will look at each individual part of HTML in much greater depth.HTML is

Most desktop applications that read and write files use a special file format. For example, Microsoft Word understands “.doc” files and Microsoft Excel understands “.xls”. These files contain the instructions on how to rebuild the documents next time you open them, what the contents of that document are, and “metadata” about the article such as the author, the date the document was last modified, even things such a list of changes made so you can go back and forth between versions.

HTML (“HyperText Markup Language”) is a language to describe the contents of web documents. It uses a special syntax containing markers (called “elements”) which are wrapped around the text within the document to indicate how user agents (eg web browsers) should interpret that portion of the document.

A user agent is any software that is used to access web pages on behalf of users. There is an important distinction to be made here—all types of desktop browser software (Internet Explorer, Opera, Firefox, Safari, Chrome etc.) and alternative browsers for other devices (such as the Wii Internet channel, and mobile phone browsers such as Opera Mini and WebKit on the iPhone) are user agents, but not all user agents are browser software. The automated programs that Google and Yahoo! use to index the web to use in their search engines are also user agents, but no human being is controlling them directly 

HTML

HTML is just a plain textual representation of content and its general meaning. For example:

<p id="example">This is a paragraph.</p>

The “<p>” part is a marker (which we refer to as a “tag”) that means “what follows should be considered as a paragraph”. Because it is at the start of the content it is affecting, this particular tag is an "opening tag". The “</p>” is a tag to indicate where the end of the paragraph is (which we refer to as a “closing tag”). The opening tag, closing tag and everything in betweeen is called an “element”. The id="example" is an attribute; you'll learn more about these later on. Many people use the terms element and tag interchangeably however, which is not strictly correct.

In most browsers there is a “Source” or “View Source” option, commonly under the “View” menu. Try this now - go to your favourite web site, choose this option, and spend some time looking at the HTML that makes up the structure of the page

The history of HTML

In the article The history of the Internet and the web, and the evolution of web standards you learned a little about how the modern Web came about. When Tim Berners-Lee invented the World Wide Web, he created both the first web server and web browser and the first version of HTML.

Whilst HTML has changed considerably since the first days, a lot of the content of modern-day HTML is embodied in that first documentation and more than half of the “tags” described in the original “HTML tags” document still exist.

As more people started writing web pages and alternatives to the original browser software, more features were added to HTML. Many were adopted universally (such as the img element used to insert an image into a document, first implemented in NCSA Mosaic). Some were more proprietary and really only used in one or two browsers. There was a growing need for standardisation — so that web developers and authors of web browsing software had a document (called a “specification”) that definitively described to them what HTML looked like so they could judge whether they were implementing/using HTML correctly.

The IETF (Internet Engineering Task Force — a standards body concerned with inter-operability across the internet) published a draft proposal of HTML in 1993. This expired without becoming a standard in 1994, but prompted the IETF to create a working group to look at HTML standardisation.

In 1995, HTML 2.0 was written, taking ideas from the original HTML draft. An alternate proposal called HTML+ was also written by Dave Raggett, which was used as a basis for many of the new elements implemented by browsers (such as the method for inserting images into documents, pioneered by NCSA Mosaic).

A draft of HTML 3.0 followed later that year, but work on that version was discontinued because of a lack of support for the direction from browser makers. HTML 3.2 dropped many of the new features of 3.0, and instead adopted many of the creations of the then-popular browsers Mosaic and Netscape Navigator.

In 1997, the W3C published HTML 4.0 as a recommendation that adopted more browser-specific extensions but also attempted to rationalise and clean up HTML. This was done by marking various elements as deprecated—which means the elements are obsolete and whilst they still exist in this version they will be removed in a later revision. This was to encourage better and more semantic use of HTML in documents (described in more detail our The web standards model).

HTML 4.01 was published in 1999, with some errata noted in 2001. This is the latest version of HTML, although HTML 5 is currently being drafted.

In 2000, the W3C also published the XHTML 1.0 specification, which was HTML re-structured to be a valid XML document.

In 2007, the W3C restarted the work on HTML by creating a new working group and adopting the work started by the WhatWG as HTML5. In this course we will be using HTML5, but don't worry — if you have already done some work in HTML4, you won't need to relearn everything. HTML5 contains all of HTML4 (albeit with some features redefined), and also adds some new powerful features on top. We'll make it clear when something we are talking about is new in HTML5.

In this article, you have learned the basics of HTML, where it has evolved from and have some insight into the structure of an HTML document. We will now continue to describe the head section of an HTML document in some more detail, before continuing to address the body content.

Note: This material was originally published as part of the Opera Web Standards Curriculum, available as 12: The basics of HTML, written by Mark Norman Francis. Like the original, it is published under the Creative Commons Attribution, Non Commercial - Share Alike 2.5 license

Android app

 Android App 

An Android app is a software application running on the Android platform. Because the Android platform is built for mobile devices, a typical Android app is designed for a smartphone or a tablet PC running on the Android OS.

Although an Android app can be made available by developers through their websites, most Android apps are uploaded and published on the Android Market, an online store dedicated to these applications. The Android Market features both free and priced apps. 

Android apps are written in the Java programming language and use Java core libraries. They are first compiled to Dalvik executables to run on the Dalvik virtual machine, which is a virtual machine specially designed for mobile devices. 

Developers may download the Android software development kit (SDK) from the Android website. The SDK includes tools, sample code and relevant documents for creating Android apps. 

Novice developers who simply want to play around with Android programming can make use of the App Inventor. Using this online application, a user can construct an Android app as if putting together pieces of a puzzle.

Networking

Networking

Networking is a process of connecting two or more computers for sharing. Through the networking, computers share information such as email, file, documents and resources such as printer, internet and disk storage. This article presents a brief overview of what computer networking is and how it works.

Introduction of Networking

Networking has single purpose and that is sharing. So if you have nothing to share, networking has nothing for you. If you anything to share, networking is everything for you. Computer networking is not a new concept. It has been here since the computers used to look like abacuses. At that time networking was used to share abacus answer with others. Over the time abacuses became computers and networking became more sophisticated. However the purpose of networking is still same: sharing the information as fast as possible. To achieve this goal networks now use electrical cables, fiber optical cables, and wireless radio signals.

A computer network includes at least two computers. Following figure illustrate a simple computer network.

A complex network may have thousands of computers connected via different communication links. For example Internet that is the largest computer network ever created by mankind.

Internet interconnects thousands of millions of computing devices including PCs, Laptops, Workstations, Server, Smartphones, tablets, TVs, Webcams, Environmental devices, Automobiles, Security cameras and many mores. In Networking all these devices are known as Hosts or End system or End devices. Computer networks use communication links to connection end devices with each other's. Communication links use different types of physical media such as coaxial cable, electrical cable, copper cable, fiber optical cable and radio spectrum. Different media types can transmit data at different rate that is measured in bits/second. Every media type has its limit that is measured in speed, distance, signal loose etc.

When a computer has data for another computer in network it initiates a session for transmission. During this process both computers finalized the rules of transmission such as speed of transmission, size of data file, security measurement of transmission, flow control etc. These rules are called protocols. Protocols control the entire data transmission through the network. Protocols are defined in various network models such as TCP/IP Layer model, OSI Layer model.

During the data transmission sender computer break the data file in small pieces. These pieces are called segment. Each segment properly wrapped with network information. Resulting segments are known as packets. Packets are sent to the destination computer through the network, where they are reassembled into the original data.

  

Requirement of Networking

very network requires specialized hardware and software to make them work. Following are the essential components for network:

Client computers

End devices that users use to access the shared resources. Usually they run desktop version of OS such as Window 10, Window 7, and Window XP. Client computers are also known as workstations.

Server computers

Computers that provide shared resources. Usually they run sever version of OS such as Window Server 8 or 2003, Linux and NetWare. Server computers run many specialized services to control the shared resources.

Network interface card

NIC is an interface that enables the computer to communicate over the network. Every computer must have a NIC in order to connect with the network. In earlier time it was a separate card and need to be installed on motherboard. All modern computers have it as the integral part of motherboard.

Communication links

Communication links are physical media. Every computer network needs some sort of media to transmit the data.

Switches

When we have more than two computers in network, we cannot connect them directly. We need a mediator device that allows us to connect all computers together. Switches do this job happily. Each switch contains a certain number of ports. We can use an eight port switch to connect eight computers.

Routers

Router is an intermediate device that speaks all language of network. It makes communication between two different networks.

Benefits of Networking

Networking is all about sharing. Networking allows us to share three main things: information, resources and applications.

Information sharing

Networking makes it easy to share the information across the network. We can send or receive data files from other computers. We can communicate with each other in network via messaging application for example email service, chat service etc. We can store data in a centralized sever for easy management.

Resources Sharing

Certain computer resources can be shared in the network such as hard disk, printer, scanner, modem etc. This allows us to track down the uses of resources. For example a network administrator can setup a printer server and share it in network. Then user can use printer server for printing. Now administrator needs only to monitor the print server instead of individual workstations.

Application Sharing

Application sharing is the most common in companies. Companies may have business application that needs to be update by several users. Sharing make it possible. It allows several users to work together on a single application.

That's all for this article. I hope you may know understand the basic terms of networking. In next articles of this section we will explorer networking concepts in more details. 

Computer virus

 A computer virus is a malicious software program loaded onto a user's computer without the user's knowledge and performs malicious actions.

Description: The term 'computer virus' was first formally defined by Fred Cohen in 1983. Computer viruses never occur naturally. They are always induced by people. Once created and released, however, their diffusion is not directly under human control. After entering a computer, a virus attaches itself to another program in such a way that execution of the host program triggers the action of the virus simultaneously. It can self-replicate, inserting itself onto other programs or files, infecting them in the process. Not all computer viruses are destructive though. However, most of them perform actions that are malicious in nature, such as destroying data. Some viruses wreak havoc as soon as their code is executed, while others lie dormant until a particular event (as programmed) gets initiated, that causes their code to run in the computer. Viruses spread when the software or documents they get attached to are transferred from one computer to another using a network, a disk, file sharing methods, or through infected e-mail attachments. Some viruses use different stealth strategies to avoid their detection from anti-virus software. For example, some can infect files without increasing their sizes, while others try to evade detection by killing the tasks associated with the antivirus software before they can be detected. Some old viruses make sure that the "last modified" date of a host file stays the same when they infect the file.

20 Common Types of Computer Viruses and Other Malicious Programs

What is Computer Virus?

Computer viruses are small software programs that are designed to spread from one computer to another and to interfere with computer operation. A virus might corrupt or delete data on your computer, use your e-mail program to spread itself to other computers, or even erase everything on your hard disk.

Computer viruses are often spread by attachments in e-mail messages or instant messaging messages. That is why it is essential that you never open e-mail attachments unless you know who it's from and you are expecting it.

Viruses can be disguised as attachments of funny images, greeting cards, or audio and video files. Computer viruses also spread through downloads on the Internet. They can be hidden in illicit software or other files or programs you might download.

How Computer Viruses Work?

Here is the general way that viruses work:

1. An infected program is run. This is either a program file (in the case of a file-infecting virus) or a boot sector program at boot time. In the case of a Microsoft Word document the virus can be activated as soon as the document that contains it is opened for reading within Microsoft Word. If the "NORMAL.DOT" document template is infected (and this is the most common target of these viruses) then the virus may be activated as soon as Microsoft Word is started up.
2. The infected program has been modified so that instead of the proper code running, the virus code runs instead. This is usually done by the virus modifying the first few instructions to "jump" to where the virus code is stored. The virus code begins to execute.
3. The virus code becomes active and takes control of the PC. There are two ways that a virus will behave when it is run: direct-action viruses will immediately execute, often seeking other programs to infect and/or exhibiting whatever other possibly malicious behavior their author coded into them. Many file-infector viruses are direct-action. In contrast, memory-resident viruses don't do anything immediately; they load themselves into memory and wait for a triggering event that will cause them to "act". Many file infectors and all boot infectors do this (boot infectors have to become memory resident, because at the time they are executed the system is just starting up and there isn't that much "interesting" for them to do immediately.)
4. What exactly the virus does depends on what the virus is written to do. Their primary goals however include replication and spreading, so viruses will generally search for new targets that they can infect. For example, a boot sector virus will attempt to install itself on hard disks or floppy disks that it finds in the system. File infectors may stay in memory and look for programs being run that they can target for infection.
5. "Malevolent" viruses that damage files or wreak havoc in other ways will often act on triggers. There are viruses that will only activate on particular days of the year (such as the infamous "Friday the 13th"), or act randomly, say, deleting a file every 8th time they are run. Some viruses do nothing other than trying to maximize their own infection to as many files and systems as .

Most Common Types of Viruses and Other Malicious Programs

1. Resident Viruses

This type of virus is a permanent which dwells in the RAM memory. From there it can overcome and interrupt all of the operations executed by the system: corrupting files and programs that are opened, closed, copied, renamed etc.

Examples include: Randex, CMJ, Meve, and MrKlunky.

2. Multipartite Viruses

Multipartite viruses are distributed through infected media and usually hide in the memory. Gradually, the virus moves to the boot sector of the hard drive and infects executable files on the hard drive and later across the computer system.

3. Direct Action Viruses

The main purpose of this virus is to replicate and take action when it is executed. When a specific condition is met, the virus will go into action and infect files in the directory or folder that it is in and in directories that are specified in the AUTOEXEC.BAT file PATH. This batch file is always located in the root directory of the hard disk and carries out certain operations when the computer is booted.

4. Overwrite Viruses

Virus of this kind is characterized by the fact that it deletes the information contained in the files that it infects, rendering them partially or totally useless once they have been infected.

The only way to clean a file infected by an overwrite virus is to delete the file completely, thus losing the original content.

Examples of this virus include: Way, Trj.Reboot, Trivial.88.D.

5. Boot Virus

This type of virus affects the boot sector of a floppy or hard disk. This is a crucial part of a disk, in which information on the disk itself is stored together with a program that makes it possible to boot (start) the computer from the disk.

The best way of avoiding boot viruses is to ensure that floppy disks are write-protected and never start your computer with an unknown floppy disk in the disk drive.

Examples of boot viruses include: Polyboot.B, AntiEXE.

6. Macro Virus

Macro viruses infect files that are created using certain applications or programs that contain macros. These mini-programs make it possible to automate series of operations so that they are performed as a single action, thereby saving the user from having to carry them out one by one.

Examples of macro viruses: Relax, Melissa.A, Bablas, O97M/Y2K.

7. Directory Virus

Directory viruses change the paths that indicate the location of a file. By executing a program (file with the extension .EXE or .COM) which has been infected by a virus, you are unknowingly running the virus program, while the original file and program have been previously moved by the virus.

Once infected it becomes impossible to locate the original files.

8. Polymorphic Virus

Polymorphic viruses encrypt or encode themselves in a different way (using different algorithms and encryption keys) every time they infect a system.

This makes it impossible for anti-viruses to find them using string or signature searches (because they are different in each encryption) and also enables them to create a large number of copies of themselves.

Examples include: Elkern, Marburg, Satan Bug, and Tuareg.

9. File Infectors

This type of virus infects programs or executable files (files with an .EXE or .COM extension). When one of these programs is run, directly or indirectly, the virus is activated, producing the damaging effects it is programmed to carry out. The majority of existing viruses belongs to this category, and can be classified depending on the actions that they carry out.

10. Encrypted Viruses

This type of viruses consists of encrypted malicious code, decrypted module. The viruses use encrypted code technique which make antivirus software hardly to detect them. The antivirus program usually can detect this type of viruses when they try spread by decrypted themselves.

11. Companion Viruses

Companion viruses can be considered file infector viruses like resident or direct action types. They are known as companion viruses because once they get into the system they "accompany" the other files that already exist. In other words, in order to carry out their infection routines, companion viruses can wait in memory until a program is run (resident viruses) or act immediately by making copies of themselves (direct action viruses).

Some examples include: Stator, Asimov.1539, and Terrax.1069

12. Network Virus

Network viruses rapidly spread through a Local Network Area (LAN), and sometimes throughout the internet. Generally, network viruses multiply through shared resources, i.e., shared drives and folders. When the virus infects a computer, it searches through the network to attack its new potential prey. When the virus finishes infecting that computer, it moves on to the next and the cycle repeats itself.

The most dangerous network viruses are Nimda and SQLSlammer.

13. Nonresident Viruses

This type of viruses is similar to Resident Viruses by using replication of module. Besides that, Nonresident Viruses role as finder module which can infect to files when it found one (it will select one or more files to infect each time the module is executed).

14. Stealth Viruses

Stealth Viruses is some sort of viruses which try to trick anti-virus software by intercepting its requests to the operating system. It has ability to hide itself from some antivirus software programs. Therefore, some antivirus program cannot detect them.

15. Sparse Infectors

In order to spread widely, a virus must attempt to avoid detection. To minimize the probability of its being discovered a virus could use any number of different techniques. It might, for example, only infect every 20th time a file is executed; it might only infect files whose lengths are within narrowly defined ranges or whose names begin with letters in a certain range of the alphabet. There are many other possibilities.

16. Spacefiller (Cavity) Viruses

Many viruses take the easy way out when infecting files; they simply attach themselves to the end of the file and then change the start of the program so that it first points to the virus and then to the actual program code. Many viruses that do this also implement some stealth techniques so you don't see the increase in file length when the virus is active in memory.

A spacefiller (cavity) virus, on the other hand, attempts to be clever. Some program files, for a variety of reasons, have empty space inside of them. This empty space can be used to house virus code. A spacefiller virus attempts to install itself in this empty space while not damaging the actual program itself. An advantage of this is that the virus then does not increase the length of the program and can avoid the need for some stealth techniques. The Lehigh virus was an early example of a spacefiller virus.

17. FAT Virus

The file allocation table or FAT is the part of a disk used to connect information and is a vital part of the normal functioning of the computer. 

This type of virus attack can be especially dangerous, by preventing access to certain sections of the disk where important files are stored. Damage caused can result in information losses from individual files or even entire directories.

18. Worms

A worm is technically not a virus, but a program very similar to a virus; it has the ability to self-replicate, and can lead to negative effects on your system and most importantly they are detected and eliminated by antiviruses.

Examples of worms include: PSWBugbear.B, Lovgate.F, Trile.C, Sobig.D, Mapson.

19. Trojans or Trojan Horses

Another unsavory breed of malicious code (not a virus as well) are Trojans or Trojan horses, which unlike viruses do not reproduce by infecting other files, nor do they self-replicate like worms.

20. Logic Bombs

They are not considered viruses because they do not replicate. They are not even programs in their own right but rather camouflaged segments of other programs.

Their objective is to destroy data on the computer once certain conditions have been met. Logic bombs go undetected until launched, and the results can be destructive

Domain name

Domain names are used to identify one or more IP addresses. For example, the domain name microsoft.com represents about a dozen IP addresses. Domain names are used in URLs to identify particular Web pages. For example, in the URL http://www.google.com/index.html, the domain name is google.com.

Every domain name has a suffix that indicates which top level domain (TLD) it belongs to. There are only a limited number of such domains. For example:

gov - Government agencies

edu - Educational institutions

org - Organizations (nonprofit)

mil - Military

com - commercial business

net - Network organizations

ca - Canada

th - Thailand

Because the Internet is based on IP addresses, not domain names, every Web server requires a Domain Name System (DNS) server to translate domain names into IP addresses