Confused about tethered jailbreak
I keep hearing about the tethered jailbreak. But all my iphones so far are untethered jailbreak. I have checked the serial numbers of the three iphones I have right now. I thought the 4th and 5th numbers determined if the iphone will be tethered or untethered. What I have read is if the 4th and 5th numbers are over 40 then it will be tethered. The phone I have now the serial starts with 5K949. That would be 49 so it should be tethered? The other two iphones I have also are above 40. Shouldn't they also be tethered jailbreak? I need to know the answer to this because all my iphones seem to be untethered. Thank you.