URL详解

非常非常好的一篇文章：

http://www.webdevelopersnotes.com/basics/urls.php3

URL stands for Uniform Resource Locator, which means it is a uniform (same throughout the world) way tolocate a resource (file or document) on the Internet. The URL specifies the address of a file and every file on the Internet has a unique address. Web software, such as your browser, use the URL to retrieve a file from the computer on which it resides.

The actual URL is a set of four numbers separated by periods(其实就是IP地址). An example of this would be 202.147.23.8 but as these are difficult for humans to use, addresses are represented in alphanumeric form that is more descriptive and easy to remember. Thus, the URL of my site which is URL 209.164.80.192 can also be written as www.simplygraphix.com. The InternetDomain Name System (就是DNS服务器) translates the alphanumerical address to numeric.

Format of a URL:
Protocol://site address/path/filename

For example, the URL of my company site is:
http://www.simplygraphix.com/
and a typical page on this site would be:
http://www.simplygraphix.com/portfolio/4.html

The above URL consist of:

• Protocol: http
• Host computer name: www
• Domain name: simplygraphix
• Domain type: com
• Path: /portfolio
• File name 4.html

Protocols

In addition to the http protocol (mentioned above), there are a few other protocols on the Internet.

• File: Enables a hyperlink to access a file on a local system. 这个也很常见啊~
  
• FTP: Used to download files from remote machines.
• Gopher: Helps in accessing a gopher server.
• mailto: Calls SMTP (the Simple Mail Transport Protocol) and enables a hyperlink to send an addressed email message.
• news: helps in accessing a USENET newsgroup.
• telnet: Provides the means for a hyperlink to open a telnet session on a remote computer.

Site Address

The site address consists of the host computer name, the domain name and the domain type. The domain name should be descriptive for easy comprehension and is usually the name of the organization or company.
There are various domain types. Some of them are listed below:

com: specifies commercial entities
net: highlights networks or network providers
org: organizations (usually non-profit)
edu: colleges and universities (education providers)
gov: government agencies
mil: military entities of the United States of America

For countries other than the U.S.A., the URL can be longer as in:
http://www.bioinfo.ernet.in

The general format of such URLs is:
machine name.domain name.domain type.country code.

Domain types can also be different for different countries. For example, an educational site can have the domain name www.school.ac.uk in the United Kingdom. Thusac (academic) is used instead of edu. Similarlycom is represented as co for Indian domain names.

Path Name

Path name specifies the hierarchic location of the said file on the computer. For instance, in http://www.simplygraphix.com/portfolio/4.html the file4.html is located in portfolio subdirectory under the server root directory.

Port

Browsers communicate with the server using entry points called ports. Associated with each protocol is a default port number, such as HTTP defaults to port 80.
The server administrator can configure the server to handle http requests at a different port. In such cases, the port number has to be supplied as a part of the URL. The port number is placed at the end of the URL after a colon.

www.some-address.com:50

HTML Anchors

With HTML you can specify anchors within files. These anchors are end points to hyperlinks placed either in the same file or some other files. Anchors are placed to link to specific locations in a file. They can be a part of the URL and are represented with a hash symbol (#) followed by the link name.

www.some-address.com/some-file.html#some-location

所以有时不需要得到请求的整个file，只需要根据偏移量得到其中需要的一部分就行了。