Domain name formulation

The right-most label conveys the top-level domain; for example, the domain name www.example.com belongs to the top-level domain com.

The hierarchy of domains descends from right to left; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example: the label example specifies a subdomain of the com domain, and www is a sub domain of example.com. This tree of subdivisions may consist of 127 levels, although more than four is rare. 

Each label may contain up to 63 characters. The full domain name may not exceed a total length of 253 characters. In practice, some domain registries may have shorter limits.

The characters allowed in a label are a subset of the ASCII character set, and includes the characters a through z, A through Z, digits 0 through 9, and the hyphen.

root domain

The root domain is at the top and is represented by period ".".

Top Level Domain

The com, edu, org and gov portions of these domain names are called the top-level domain or first-level domain. There are several other top-level domain names, including MIL, NET as well as unique two-letter combinations for every country also called Country Code Top-Level Domain (ccTLD). For India the ccTLD is ".in".

Second Level Domain

Within every top-level domain there are several Second-Level Domains. For example, in the COM first-level domain, you've got:

•   opensourcenuts
•   microsoft
•   redhat
•   yahoo
•   plus millions of others

Every Second-Level Domain name in the top-level domain must be unique, but there can be duplication across domains. For example, opensourcenuts.com and opensourcenuts.org are completely different domain name. 

Second level domains are registered to individuals or organisations. 

In some countries few Second-Level Domain are reserved. For example in India domain name are categories as under:

• .co.in (originally for banks, registered companies, and trademarks)
• .firm.in (originally for shops, partnerships, liaison offices, sole proprietorships)
• .net.in (originally for Internet service providers)
• .org.in (originally for non-profit organizations)
• .gen.in (originally for general/miscellaneous use)
• .ind.in (originally for individuals)

Six zones are reserved for use by qualified organizations in India:

• .ac.in (Academic institutions)
• .edu.in (Educational institutions)
• .res.in (Indian research institutes)
• .ernet.in (Older, for both educational and research institutes)
• .gov.in (Indian government)
• .mil.in (Indian military)
• .nic.in is reserved for India's National Informatics Centre, but in practice most Indian government agencies have domains ending in .nic.in.

Third Level Domain

Third Level domain, unlike a domain name, is not registered anywhere because it is associated with a domain name only. It can be created by the web host on the DNS server. Third Level domain are commonly used to categorize portions of the website. sub-domain can be a separate site on the same server. Sub-domain can point to a sub-directory on the same server. Sub-domain can be a separte site on different server which may reside at different geographical location.