Network problems between Windows 7 and XP

I have 2 pc's in a netwerk and a SMC router (4 ports and Wireless)
Both computers are connected to the Router with cable and wireless.
From Both computers I can acces the internet (Router is also gateway)
So I have 4 IP-adresses:
- XP (wireless)
- XP (LAN)
- W7 (Wireless)
- W7 (LAN)

When I do a ping from the XP pc to the windows 7 PC (..101 or 102) I get a reply from the network adapter of the WIndows 7.
But when I do the same on the Windows 7 PC to the XP pc (..100 or 103) I don't get a reply.
Why ??

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