Why Internationalization?
With the Internet fast speed developing, more and more people aware of the importance of Internet. I am sure that everybody may very familiar with web addresses, which are used to indicate resources on the web. Traditional web addresses are formed by only ASCII characters, such as a-z, A-Z, and so on. Nowadays, a multilingual web address is showing up on the internet, this kind of web address are not just containing ASCII characters, it could be formed by all kind of Unicode, such as Chinese language, Russia languages. But how could this happen? How does it work? Following I will illustrate these issues in details.
Traditional web addresses are represented by uniform resource identifiers, which is URI in short. RFC 3986, the generic syntax for URI, defines a grammar that is a superset of all valid URIs, allowing an implantation to parse the common components of a URI reference. The characters that RFC 3986 allowed are only English languages, European numerals and a small number of symbols. This explains why traditional web addresses could only be formed by ASCIIs. In order to satisfy users could use their native languages in web addresses, here comes another identifier, Internationalized Resource Identifiers, which making URI system more worldly and accessible.

Previous Knowledge---Uniform Resource Identifiers
Uniform Resource Identifiers, which also called URI, are used for expressing traditional web addresses. Web addresses indicated by URI are limited to only ASCII characters, such as lower and upper A to Z letters, and some symbols.

Brief Introduction to IRI
International Resource Identifiers is a superset of URI; it is a sequence of characters from the Universal Character Set, not only ASCII character set. RFC 3987, the syntax for IRI, this kind of approach defining a new protocol element was chosen instead of extending or changing the definition of URIs. This was done in order to allow a clear distinction and to avoid incompatibilities with existing software.
For a simple IRI, it always contains three parts, the scheme, the domain name and the path that point to the location of the resource. Such as Figure 1 shows to us and following we will focus on the solutions for domain names.
Figure 1:

Internationalized Domain Names---IDN
Internationalized Domain Names, such as 华人.公司.cn, is an Internet domain name that contains at least one label that is displayed in software applications. Internationalized domain names are stored in the Domain Name System as ASCII strings using Punycode transcription.

The Standard for IDN---IDNA
In order to fulfill the requirements of end users who want to read and write with their own languages, which maybe non-ASCII characters, Internationalizing Domain Names in Applications (IDNA) was selected as the standard. Although the research for IDN has been started since early 1990, the initial versions for IDNA standard were published at the year 2003 and after several years implantation and examination, IDNA standard has been updated at the year 2008.
Internationalizing Domain Names in Applications as a standard was used to handle IDNs. From the document RFC 3490, we could get that IDNA works by allowing applications to indicate Unicode name labels through the existing ASCII name labels. Since IDNA does not need to change any infrastructure, there will be no impacts to the lower-layer, and lower-layer protocols do not need do any change to accommodate to this standard.