IRC USER message makes no sense to me

First, please tell me if I'm not allowed to ask about this protocol here... It just seemed like the best place. Sorry if I'm wrong.

So according to wikipedia the USER message goes like the following:

USER username hostname se开发者_StackOverflowrvername :realname

What's the difference between hostname and servername? Thanks.

RFC 1459 states in section 4.1.3:

Note that hostname and servername are normally ignored by the IRC server when the USER command comes from a directly connected client (for security reasons), but they are used in server to server communication.

USER command from RFC1459 is modified in RFC2812 which moots your question slightly.

The "hostname" is that of the client.