chapter 1

A URL is composed of several parts:

1)Scheme - Characters at the beginning of a URL followed by a colon ":" or a colon and double slashes "://". Common URL schemes include http, https, mailto, and file. Ex: In http://www.cdc.gov/alcohol, the scheme is "http". 2)Hostname - The complete domain name specified in the URL. Ex: In http://www.cdc.gov/alcohol, the hostname is "www.cdc.gov". 3) Path - All characters to the right of the hostname in the URL. Ex: In http://www.cdc.gov/alcohol, the path is "/alcohol".

An HTTP request and an HTTP response are both composed of four parts:

1. Status line 2. Zero or more header fields 3. Empty line 4. Optional message body The web server is able to identify the requested resource from the path in the status line. The web server sends back the requested resource in the message body of the response.

website

A collection of related web pages are organized into a website and stored on a web server.

top-level domains (TLD)

A domain name belongs to one of numerous top-level domains (TLD), such as .com, .net, .org, .edu, and .gov.

link

A link on a web page is a clickable item that causes the web browser to jump to another web page when clicked. Ex: Click here is a link to an interesting website. A button or image can also have a link.

browser

A program for viewing web pages.

digital certificate

A website wanting to use HTTPS must acquire a digital certificate issued by a trusted certificate authority, which contains a public key needed to encrypt data between the browser and web server.

country code top-level domain (ccTLD)

Also, each country is assigned a unique two-letter country code top-level domain (ccTLD) like .uk (United Kingdom), .ru (Russia), and .de (Germany).

request method

An HTTP request method indicates the desired action to perform on a resource.

status code

An HTTP response status code is a three digit number that indicates the status of the requested resource.

information mesh

Another name for the web that was considered by the web's creator.

domain name registrar

Anyone may register an unused domain name with a domain name registrar. Most registrars charge a yearly fee for keeping the domain registered. Once a domain name is registered, the owner's name, address, and other registration information is made publicly available from ICANN's Whois service.

HTTP request and response headers.

Content-Length: Number of bytes in the response's message body. Content-Type: Media type of the response's message body. Date: Datetime the response was generated by the web server. Last-Modified: Datetime the requested resource was last modified on the web server. Server: Identifies the web server software that generated the response. Host: The domain name for the requested path. User-Agent: Identifies the browser making the request.

DOM

Document Object Model

Javascript

Enables document interaction is a programming language that runs in a browser, enabling web pages supporting actions like responding to a button click. JavaScript can be included in the HTML file's header part.

FTP (File Transfer Protocol)

FTP (File Transfer Protocol) was an early way for transferring files over the Internet. Users used FTP programs to connect to servers, look at listings of available documents, and download documents of interest.

IoT devices display one or more of the following characteristics:

Gather information about the physical world using sensors. Ex: temperature, voltage, images. Share the sensor data with control systems. Ex: A sensor may report a room's temperature to building control system, or air ozone and water pollution sensors reporting air quality data to urban environment monitoring systems. Interact with hardware to execute commands sent by control systems. Ex: Speed up fan moving hot air to part of a building where the sensed temperature is below the target temperature.

second-level domain

Immediately after a top-level domain comes a second-level domain, such as wikipedia in wikipedia.org. A second-level domain is commonly an organization's name as in Stanford.edu, or indicates the purpose of a website as in DoPython.org. ex: .gov

CSS

Manages document presentation is a textual language for describing how a web page is styled for visual presentation. CSS controls the look and layout of web page content.

Important mobile development topics.

Mobile First : A web development approach that advocates first creating a reduced-feature version of a website for mobile users. Then, the developer creates a full-featured website for visitors using desktop computers. Screen size: Physical dimension in pixels of a device's screen. Affordances: Visual clues that guide the user in figuring out how to use an app. Responsive web design: A web design approach that creates web pages that automatically move and resize parts depending on the display size and orientation.

Developers of mobile-friendly web pages need to consider the following concerns:

Screen size: Screen size is much smaller than desktops. Load speed: Mobile devices may have limited or slower Internet connectivity. Device speed: Limited memory and CPU speed of mobile devices means mobile browsers are not as powerful as desktop browsers. Data cost: Many users have data plans that limit how much content can be downloaded. Large web pages and web pages that keep requesting more data reach a mobile data plan's limit faster. Battery life: A web page that constantly runs JavaScript and frequently loads data will drain the battery faster. Interface: User interaction is with touch, not a mouse.

Common HTTP response status codes.

Status code Status phrase Meaning 200 OK Standard response for a successful request. 301 Moved Permanently The resource should always be requested at a different URL. 302 Found The resource should temporarily be requested at a different URL. 304 Not Modified The resource has not been modified since the last time the resource was requested. 403 Forbidden The web server does not have permission to access the resource. 404 Not Found The resource could not be located. 500 Internal Server Error Something unexpected happened on the web server.

Internet

The Internet began as four networked computers in 1969 but is today the largest and most popular computer network, spanning the entire globe. Early versions of the Internet looked and acted similar to today, but in a much simpler way. Documents were plain text: no images, no formatting, and no interaction.

IPv4

The original Internet Protocol, known as IPv4, has 32-bit addresses. 32 bits can represent 232 or about 4 billion unique addresses, originally believed to be more than would ever be needed;. But 4 billion is no longer enough

Early 1990's

The period when the web was first developed.

HTTP

The protocol for transferring files among computers via the web.

World Wide Web, or simply "the web".

The web involved three things: 1)Text files, known as HTML files, containing links to other text files. 2)A program, known as a browser, for viewing HTML files. 3)A set of rules, known as the HTTP protocol, for transferring HTML files among computers.

conditions affecting web accessibility include:

Visual problems like blindness, low vision, or color blindness Hand control issues ranging from tremors to total inability to use hands Seizures caused by flashing on screen Cognitive challenges like dyslexia and other difficulties in processing web pages

When web pages are built so as to cleanly separate document structure, visual layout, and interaction, the following results are obtained:

Web pages are more likely to work as intended across a wide range of browsers. Web pages work better on smartphones and other mobile devices. Internet of Things systems are easier to build because relevant data is easier to identify. Web accessibility is improved. Intelligent systems can extract more meaning from the content of web pages.

DNS server

When a computer sends a packet using a domain name over the Internet, the first step is to contact a DNS server to convert the domain name to an IP address.

Mobile devices

are handheld computers such as smartphones and tablets

Common CSS properties.

background-color:Element's background color ex: h1 { background-color: lightgray; } font-family Font used for the element's text. ex: p { font-family: arial; } font-size: Font size used for the element's text. ex: p { font-size: 9pt; } padding: Spacing between element's content and border. ex: strong { padding: 5px; } margin: Spacing around element (outside element's border). ex: p { margin-left: 10px; } Note: margin-top, margin-bottom, margin-left, and margin-right specify margin for individual sides.

CSS color

can be a pre-defined name as in color:blue, or an rgb value as in color:rgb(50, 100, 255).

root servers

hirteen main DNS servers (called root servers) exist in the world, and a computer's operating system or an ISP keeps a reference to the root servers' IP addresses. Ex: 198.41.0.4 (run by Verisign), 192.228.79.201 (run by USC), 199.7.91.13 (run by Univ. of Maryland), and 192.203.230.10 (run by NASA).

web server

is a computer (or software running on a computer) that serves web pages to web browsers.

IP address

is a computer's unique address on the Internet (like a house's unique address in the world), usually represented numerically like 198.51.100.7.A typical IP address is 32 bits, divided into four 8-bit groups, each group often written as a decimal number.

web page

is a document that is viewed in a web browser.

HTTP request

is a message sent from the web browser to the web server. Often the request asks the web server to send back a web resource like an HTML file, image, CSS style sheet, JavaScript file, or video.

HTTP response

is a message sent from the web server back to the web browser in response to an HTTP request. Often the response contains the requested web resource.

domain name

is a name for an IP address, such as the name wikipedia.org for the IP address 208.80.154.224; the name is easier to remember and type.

JavaScript function

is a named group of statements that can be run by referring to that name.

HyperText Transfer Protocol (HTTP)

is a networking protocol that runs over TCP/IP and governs communication between web browsers and web servers.

web browser

is a program that downloads an HTML document from a server, displays the document to the user with the appropriate formatting, and allows the user to interact with the document, such as clicking hyperlinks to access other documents. A web browser uses HTML to understand the structure and semantics, or meaning, of the document.

Transmission Control Protocol/Internet Protocol (TCP/IP)

is a protocol suite that governs how packets of data are transferred over the Internet from one machine to another. Understanding the details of TCP/IP is not usually required of web developers, but a thorough understanding of HTTP is necessary to create effective web applications.

HTTPS

is a protocol that encrypts HTTP traffic between a browser and web server so a network sniffer cannot intercept sensitive information in the HTTP traffic like passwords, credit card numbers, financial transactions, etc.

URL shortening

is a technique to create shorter URLs that redirect to longer URLs. Ex: http://en.wikipedia.org/wiki/URL_shortening has a short URL of http://tinyurl.com/urlwiki.

browser cache

is an area on the computer's disk where web content can be stored by the web browser for quick retrieval later. By caching web content, browsers can reduce the amount of network traffic required to display previously visited web pages.

entity tag (ETag)

is an identifier for a specific version of a web resource. Ex: 34905a3e285dd11.

web resource

is any retrievable item, like an HTML file, image, video, CSS style sheet, etc.

DNS

is short for Domain Name System.

rgb

is short for red, green, blue; all colors can be formed by combining those three colors. Values range from 0 (none) to 255 (bright).

network sniffer

is software that monitors network traffic and allows users to inspect HTTP requests and responses.

markup

is special markings in the document that provide additional information about links, formatting, and images.

Hypertext

is text that has links to other text (and today to images, videos, and more).

Web accessibility

is the ability of users with disabilities to access and use a web page with reasonable effort. Designing accessible web pages ensures equal access and opportunity for everyone.

Separation of concerns

is the design principle of breaking up web content using distinct languages and documents that overlap as little as possible. In modern web design, document structure and text, visual layout, and page interaction are separately specified using three key languages. HTML is the language that describes the page content. CSS is the language that describes page layout. JavaScript is the language that provides interactive functionality.

Linkrot

is the general name for once valid links that now return 404 status codes.

Internet of Things

is the global collection of communicating devices that sense and control technology on behalf of humans. IoT devices range from a simple temperature sensor to a satellite-based laser scanner used to discover archaeological sites hidden by vegetation.

URL (Uniform Resource Locator)

is the location of a web resource on the web, such as http://www.cdc.gov/alcohol/faqs.htm.

HyperText markup language (HTML)

is the standard markup language for web documents. The language used for a text file that describes a web page. Specifies document structure is a textual language for creating web pages.HTML files are usually saved with a .html or .htm file extension. Ex: index.html. A

Cognitive computing

is the use of artificial intelligence techniques and access to vast amounts of data to simulate human problem solving in complex situations with ambiguity, changing data, and even conflicting information.

browser redirect

is when the web server returns a 301 or 302 status code with a Location header indicating the URL the browser should load next.

Third-level and further level domains

refer to sub-computer systems local to an organization, as in cs.stanford.edu where the cs is for Stanford's Computer Science department. A common third-or-deeper-level domain is www, short for World Wide Web, usually referring to an organization's web server. Many organizations use www optionally, so stanford.edu by default goes to www.stanford.edu.

Common HTTP request methods.

request method GET-Request a representation of the specified resource. HEAD-Request a response identical to GET but without the response body. POST-Request the web server accept the message body enclosed in the request as a new resource. PUT-Request the web server accept the message body enclosed in the request as a modification of an existing resource. DELETE-Request the web server delete the existing resource.

CSS rule

selects specific HTML elements and specifies styling properties for the element.

Steps in an HTTPS transaction.

step 1 : Browser requests an HTTPS to a web page. step 2 : Web server sends a digital certificate to the browser step 3 : The browser and web server initiate an SSL handshake. step 4: The browser and web server transmit encrypted information.

variable

stores a value or a link to an element of a web page. Using the link stored in a variable allows JavaScript to directly modify the properties of an element defined elsewhere in the HTML document. Ex: x stores the element with id="Colorable".

W3C

the World Wide Web Consortium) is the international standards organization that controls web standards.

ICANN

the organization that manages TLDs, now allows companies and organizations to create customized TLDs, like .church, .pizza, and .music.

DNS lookup

the web browser performs a DNS lookup by sending the domain name to the local DNS and getting back the IP address of the web server hosting the domain name. Ex: https://www.w3c.org/ has a domain name of www.w3c.org, and DNS translates www.w3c.org to the IP address 128.30.52.45. Before a TCP connection is created, the web browser performs a DNS lookup

IPv6

uses 128-bit addresses, capable of representing 2128 addresses. That number is or 3.4 × 1038 or 340,000,000,000,000,000,000,000,000,000,000,000,000 hopefully enough for quite a while. IPv4 and IPv6 currently co-exist, and likely will for a long time.

404 error message

which is a code number for page not found. Various other codes exist. Many web servers return a page that includes the number "404".