HTTP The Definitive Guide
按址标识 identify by address
按名标识 identify by name
Domain Name Server
Uniform Resource Name Server
https://en.wikipedia.org/wiki/Ln_(Unix)
The ln command is a standard Unix command utility used to create a hard link (link) or a symbolic link (symlink) to an existing file.[1] The use of a hard link allows multiple filenames to be associated with the same file since a hard link points to the inode of a given file, the data of which is stored on disk. On the other hand, symbolic links are special files that refer to other files by name.[2]
/*
符号链接 软链接 指向另一个不同路径文件的一个符号路径
硬链接 一个存储了链接建立时它所指向文件的实际数据的文件副本
原文件删除后，符号链接将失效，并且访问软链接时，会提示找不到文件，但硬链接文件还在且保存有原始文件的内容。
*/
uniform resource identifier URI 统一资源标识符
uniform resource locator URL 统一资源定位符
uniform resource name URN 统一资源名
A URN serves as a unique name for a particular piece of content, independent of where the resource currently resides. These location-independent URNs allow resources to move from place to place. URNs also allow resources to be accessed by multiple network access protocols while maintaining the same name. For example, the following URN might be used to name the Internet standards document "RFC 2141" regardless of where it resides (it may even be copied in several places):
urn:ietf:rfc:2141
URNs are still experimental and not yet widely adopted. To work effectively, URNs need a supporting infrastructure to resolve resource locations; the lack of such an infrastructure has also slowed their adoption. But URNs do hold some exciting promise for the future.
URLs are a powerful tool. Their design allows them to name all existing objects and easily encompass
new formats. They provide a uniform naming mechanism that can be shared between Internet
protocols.
However, they are not perfect. URLs are really addresses, not true names. This means that a URL tells
you where something is located, for the moment. It provides you with the name of a specific server on
a specific port, where you can find the resource. The downfall of this scheme is that if the resource is
moved, the URL is no longer valid. And at that point, it provides no way to locate the object.
What would be ideal is if you had the real name of an object, which you could use to look up that
object regardless of its location. As with a person, given the name of the resource and a few other
facts, you could track down that resource, regardless of where it moved.
The Internet Engineering Task Force (IETF) has been working on a new standard, uniform resource
names (URNs), for some time now, to address just this issue. URNs provide a stable name for an
object, regardless of where that object moves (either inside a web server or across web servers).
Persistent uniform resource locators (PURLs) are an example of how URN functionality can be
achieved using URLs. The concept is to introduce another level of indirection in looking up a
resource, using an intermediary resource locator server that catalogues and tracks the actual URL of a
resource. A client can request a persistent URL from the locator, which can then respond with a
resource that redirects the client to the actual and current URL for the resource (see Figure 2-6). For
more information on PURLs, visit http://purl.oclc.org.