I've got a problem. Since a few days my Mac Air can't connect to internet because Airport autoassign ip. I have DHCP on, Refresh DHCP, Check and repair Hard Disk permissions ... It doesn't work. I must introduce manually th IP and router's IP if I want to use internet, but when I go outside and use another conexion, I can't.

Can someone help me, please?

i had the same problem a while ago but it just kinda went away. My best guess would be to restart and zap the ram. Hold down command-option-p-r until you hear the startup chime twice