Real IPV4 addresses are getting scarse, yet very few people have moved over to IPV6. This is mainly by hiding hundreds of computers on a private network behind a single real IP. These machines can be assigned IPs from the netblocks listed in this standard.
This is version 1.0 of the standard that most of what is popularly known as "The Internet" uses as a transport mechanism. These days most implementations are HTTP/1.1 though.
This is a far more advanced version of the protocol specified in RFC 1945.
This standard describes a way of delegating reverse lookups for IP address to the people who control them, in smaller blocks than were supported before. I have not seen many providers actually implementing it, partially because users tend not to care what their address resolves to. This standard has one minor disadvantage of liking "/" characters in the domains, meaning that having the filename containing the nameserver config for you domain in a file named after the domain is difficult on Unix systems.
eMail, the so-called killer app of the Internet. How does it all work? How does an email manage to get to the right person most of the time? Well if people actually followed the rules in this doccument it would always either get delivered or the sender would get an error message. RFC 821 is also required reading for anyone implementing a mail transfer agent.
This is the standard for the layout of the message content for emails and news articles. Even less softwares appears to give a shit about the content of this one. See also RFC 822.
This is getting less interesting. In order to cryptographically sign an XML document, it is probably a good idea to do so after parsing it, so that you just sign the meaning. This standard describes how to represent an XML document in the same way as other people.