Some Android apps (and hacked iPhones) like PDA Net would tweak the TTL so that when you tethered your laptop to your phone, the data packets looked like they originated from your phone instead of taking an extra hop from your laptop.

This trick served two purposes. First, it hid the fact that you were tethering a laptop. Second, it let you use your phone’s unlimited data instead of tapping into your limited hotspot allowance.

Many carriers cap your hotspot data at, say, 10 GB while giving your phone unlimited data. By adjusting the TTL value, all connected devices appear to share the phone’s TTL, so you end up using the phone’s unlimited data.

I may or may not have used this internationally long ago when my plan included unlimited phone data abroad but excluded any hotspot data. With PDA Net, I could tether to my heart’s content for free.