send_arp禁止上网试验不成功！

用send_arp禁止一台机器Windows XP通过网关试验，
Windows XP机器IP:10.55.56.165  MAC: 00:0a:ff:ef:18:42，网关IP: 10.55.56.251 MAC 00:0a:ff:ef:18:66，用send_arp发送：
send_arp 10.55.56.165 00:0a:ff:ef:18:44 10.55.56.251 00:0a:ff:ef:18:66 400
用Windows XP机器上网，仍然正常，请问斑竹为什么？