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In these world nothing is secure...u cant secure your network im sorry, u will always find a hacker that can crack in your network (especialy if he's useing linux) but u can discourage lame hacker or good hackers that dont want to waste a lot of time with this simple rules:

1)Change your router username,password and SSID
2)Use advance encryption: Wpa-PSK (TKIP) is supported by a lot of wifi devices, but its better if u use more advance encryptions (!!!DONT USE WEP!!! IT TAKES 15MIN TO CRACK USEING WINXP)
3)Use the MAC adress filter.
4)Dont broadcast your SSID
5)Use a network monitor utility i suggest look@lan (www.lookatlan.com - Network Monitoring and Management Solutions - Network Monitoring and Management Solutions)
6)Turn off DHCP on AP
7)IMPORTANT turn off your network when yuor not useing it.