DOMAIN 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.
The IP address for eos.cs.berkeley.edu. would be stored at the eos node.
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). Underscores are not valid.
Microsoft 'WINS' and 'ACTIVE DIRECTORY' records contain UNDERSCORE characters. Underscore characters are not part of the Domain Name Service Standard (RFC's 1034 and 1035). UNDERSCORES WILL SCREW UP YOUR DNS and cause it to FAIL when a zone transfer is attempted by a Non-Microsoft machine. The UNDERSCORE character is not valid in DNS.
FULLY QUALIFIED DOMAIN NAMES (FQDN)
Fully qualified domain names (FQDNs) are names that have been spelled out all the way to the root of the DNS hierarchy. Using the example DNS hierarchy diagram above, the true, fully qualified domain name for the host eos would be eos.cs.berkeley.edu. (note the trailing dot after the edu in the name). Most DNS resolvers are smart enough not to need the last dot and it is assumed to be there; however, when troubleshooting domain name service issues with applications such as nslookup, its a good idea to use the period at the end.