HTML and CSS Reference
In-Depth Information
time there was updated information to display, the user or browser had to refresh an
entire page to fetch a complete resource by using HTTP.
With JavaScript and the XMLHttpRequest API, a set of techniques called AJAX were
developed to allow more seamless applications that did not have abrupt transitions
during every interaction. AJAX let applications fetch just the resource data of interest and
update a page in place without navigation. With AJAX, the network protocol is still HTTP;
the data is only sometimes, but not always, XML despite the XMLHttpRequest name.
The Web has become pretty popular. So popular, in fact, that many confuse the
Web with the Internet since the Web is often the only significant Internet application
they use. NAT (Network Address Translation), HTTP proxies, and firewalls have also
become increasingly common. Today, many Internet users do not have publicly visible
IP addresses. There are many reasons why users do not each have unique IP addresses,
including security measures, overcrowding, and simple lack of necessity. The lack of
addresses prevents addressability; for example, worms that require public addresses
cannot access unaddressed users. Additionally, there are not enough IPv4 addresses for
all Web users. NAT allows users to share public IP addresses and still surf the Web. Finally,
the dominant protocol, HTTP, does not require addressable clients. HTTP works fairly
well for interactions driven by client applications, since the client initiates every HTTP
request, as shown in Figure 3-2 :
Figure 3-2. HTTP clients connected to a Web server
Essentially, HTTP made the Web possible with its built-in support for text (thus
supporting our interconnected HTML pages), URLs, and HTTPS (secure HTTP over
Transport Layer Security (TLS)). However, in some ways, HTTP also caused the Internet
to regress due to its popularity. Because HTTP does not require addressable clients,
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