Overview of TCP/IP on wikipedia. You don't need
to know the history or details of the layers, but you need to know
what the layers are and the purpose of each one.
Ajax:
A New Approach to Web Applications - an article by Jesse
James Garrett, one of the founders of the AJAX approach who was the
first one to come up with the term.
A nice short article on different ways of
processing XML using JavaScript. Covers the XMLDOM approach (plain
JavaScript) and a combination of CSS and XSLT processing. Firefox only
(though presumably
XSLT processing and DOM work similarly in other browsers).