DNS/1/domainについて、ここに記述してください。

どこで、どう定義されているかが重要なのではなくて、

それができない・分っていないひとが多い。-- ToshinoriMaeno 2019-08-04 22:09:52

RFC 1034

"domain name" 

2. INTRODUCTION

This RFC introduces domain style names, their use for Internet mail and
host address support, and the protocols and servers used to implement
domain name facilities.

2.1. The history of domain names

The terms "domain" or "domain name" are used in many contexts beyond the DNS described here.

Very often, the term domain name is used to refer
to a name with structure indicated by dots, but no relation to the DNS.
This is particularly true in mail addressing [Quarterman 86].

domain, subdomain

3.1 

A domain is identified by a domain name, and consists of that part of
the domain name space that is at or below the domain name which specifies the domain.

A domain is a subdomain of another domain if it is contained within that domain.
This relationship can be tested by seeing 
if the subdomain's name ends with the containing domain's name.
For example, A.B.C.D is a subdomain of B.C.D, C.D, D, and " ".

subdomain

3.4

In this example, the root domain has three immediate subdomains: MIL, EDU, and ARPA.

The LCS.MIT.EDU domain has one immediate subdomain named XX.LCS.MIT.EDU.

All of the leaves are also domains.