Networking for Dummies?

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It's time for me to look into multiplayer networking over a lan (it doesn't need to run over the internet for now, trust me on this), but I know nothing about network protocols, so...

Blitz implements DirectPlay - that looks pretty comprehensive. So do I even need to bother looking at the TCP or UDP commands?
Can someone explain simply the effective reasons for having three different systems.
I guess the most complicated thing I will need to do is (predictably) alert each PC as to the movements of players on the other PCs.
Thanks.