username@some.machine.nameEverything to the right of the @ sign identifies the computer on the Internet that will receive the mail. For example, your e-mail address on the OCF has ocf.Berkeley.EDU following the @ sign. In most cases, it doesn't matter if letters are upper case or lower case.
Names identifying computers on the Internet are composed of several words separated by periods. These words specify where a computer resides in the Internet hierarchy, and are read right-to-left. The rightmost 2 words identify the domain in which the computer is located.
The rightmost word identifies what sort of organization the computer is owned by or the country in which it is located. For instance, names ending in .edu are in the educational domain (Colleges and Universities), a name ending in .com is in the commercial domain, and a name ending in .mil is in the military domain. Most sites outisde the United States are in domains named after the country. Examples include .uk (United Kingdom), .jp (Japan), and .ca (Canada).
Each word after the domain as the address is read from right to left specifies a further subdomain, and the leftmost one specifies the actual name of the computer. For example, consider the computer named:
cory.EECS.Berkeley.EDUThe machine is in the educational domain since it ends in .edu. The machine is somewhere at the University of California at Berkeley since the next part is the world Berkeley. The next subdomain listed is EECS, which is the department of Electrical Engineering and Computer Science at UC Berkeley. Finally, the name of the machine in the EECS department at Berkeley is cory.
So, consider a full e-mail address, such as:
kenji@ocf.Berkeley.EDUThe computer is at the University of California at Berkeley, and it's name is ocf, which is the Open Computing Facility. At that machine, this address indicates that mail should go to the user at the OCF named kenji.
However, there are several other ways this address might be written. If you were sending me mail from somewhere else in Berkeley, (that is, if the name of your computer also ended in .Berkeley.EDU), you could leave out that part and simply write the address as:
kenji@ocfFurthermore, if you were on the same computer (on the OCF), then everything after the @ sign could be left out, leaving simply:
kenjiIf you write an address that is incomplete (i.e., it doesn't end with a top-level domain name such as .edu, .com, or .uk for example), then everything after where you left off is implicitly assumed to be the same as in the name of the computer you are using.