Peer to Peer ... ... on a Lan

Again, unicasting (TCP or UDP) is possible
but not at all adapted (not scalable).
Broadcast
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Each packet is received (and read) by all players (and computers) on the
LAN : busy LAN and busy computer !
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Floods the network with packets.
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All packets must be brought up through the kernel of the operating systems
of all players on the net.
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Even if the packet is not for that machine!
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Thereby wasting CPU time.
NB. in WarCraft, we need a server even on a LAN because of Licensing issue
(the CD is the key !)