[xubuntu] Router network: XP PC able to see Xubuntu PC, Xubuntu does not see XP PC !

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http://ubuntuforums.org – I have a Dlink router. One XP PC connected to it. IP= One Xubuntu PC connected to it. IP= Both PC access Internet. My XP PC sees my Xubuntu PC. Pinging works. My Xubuntu PC does not see/reach my XP PC. Pinging does not work, unreachable. I have no filters at all, default configuration. I have no sharing setup. I have no firewall. How can I have both PC be able to see each other ? (HowTos)