Peer to Peer ...   ... on a Lan

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