Method #1
1) Turn off Wi-Fi
2) Open Network Preferences:
3) Click "Advanced"
4) Delete the ISTWL-"Name" using the +/- button:
5) Click "OK", then"Apply"
6) Turn on Wi-Fi
Method #2
1) Turn off Wi-Fi
2) Open Network Preferences:
3) Click "Advanced"
4) Delete the ALL Wi-Fi networks using the +/- button:
5) Click "OK", then "Apply"
6) +/- create Wi-Fi
7) Click "Apply"
8) Turn on Wi-Fi