The server netcode is a bit of a mess to read. I made a start at a
client networking library a little while ago that you might find a bit easier to follow. Doesn't have many of the details filled out (basically all it can do at this point is connect to a server masquerading as your client of choice, then send some chat messages and not much else) but the fundamentals are there.
Socket ->
Ack Manager ->
Protocol Receiver probably the best places to start.
One thing I'd like to say about EQ's networking is that it kinda sucks. Plenty of one-off oddities and conditionally-present flags that seem like things that crept in over time and were never properly incorporated into the design. Could have saved some headaches if they just reserved an extra byte or two in the per-packet header for some 1-bit flags.