DNS Names
A Domain includes all devices that fall under a particular part of the Domain Name System hierarchy. For instance, the domain name inetdaemon.com would be used for all computer names that belong to InetDaemon. Basically, a domain name is used as the identifier for a group of computers that as far as DNS are concerned are all part of the same group.
DOMAIN NAME HIERARCHY
Domain Names are hierarchical. Typically, in the DNS hierarchy, you will have the following:
- Top Level Domains (TLD's)
- Second Level Domains
- Sub-Domains
- Host Name
Within the hierarchy, you will start resolution at the top level domain, work your way down to the second level domain, then through zero, one or more sub domains until you get to the actual host name you want to resolve into an IP address.
It is traditional to use different DNS servers for each level of the hierarchy. The Top Level domain is handled by the DNS servers at InterNIC, InterNIC points to the second level domain name servers, if there are sub-domains, each domain's server will delegate to the DNS server at the next lower level in the hierarchy.
For example, in the figure below, .edu is the top level domain, berkeley is the second level domain, and .cs is the sub-domain of berkeley. Eos is the host name.
The IP address for eos.cs.berkeley.edu. would be stored at the eos node.
DOMAIN NAMES
Domain Names as supported in the Domain Name System must be less than 63 characters in total length, begin and end with a printable character, and can contain only letters, numbers and the hyphen character (the hyphen '-' must be in the middle somewhere).
The UNDERSCORE character is not valid in DNS. Microsoft 'WINS' records are not part of the Domain Name Service Standard (RFC's 1034 and 1035), and in addition contain UNDERSCORES. UNDERSCORES WILL SCREW UP YOUR DNS and cause it to FAIL when a zone transfer is attempted by a Non-Microsoft system (such as BIND, the most common DNS server on the planet).
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