[Web] Domain name, Sub domain name

□ Domain name

○ Character representation Address of IP address(x.x.x.x → mail.google.com)
○ Each label(mail, google, com) is separated by full stop(.)
○ Right end label is called TLD(Top-level domain)
○ Left label of TLD is called "SLD(Second-level domain)"

□ Sub domain name

○ Left domain names of the TLD

- Ex) "mail.google.com" is sub domain of "google.com"
- Ex) "google.com" is sub domain of "com"

○ In general, the sub-domain contains relevant product or service

- Ex) mail.google.com → e-mail service provided by google