Two servers on the same network can't ping eachother

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http://serverfault.com – According to this guide I setup two servers. Now these two servers can't ping each other, this is arp -n result: # arp -n Address HWtype HWaddress Flags Mask Iface x.x.x.129 ether xx:xx:xx:xx:xx:xx C br0 x.x.x.21 (incomplete) br0 x.x.x.138 (incomplete) br0 I can't ping x.x.x.21 Note: according to that guide br0 is default interface and there is no MAC Address Defined in it. So How can I fix it? Thanks (HowTos)