Just the toes or the whole foot?

[Adam]

I was watching the Nebraska/Missouri game and saw the would-be-touchdown ruled incomplete because the receive didn't have his foot down in bounds. Replays showed he landed on his toe and kept the heel off the ground for a moment, maintaining control the whole time, before falling backwards out of bounds.

His toe was inside the line, but once the heel came down it was on the line.

They reviewed it in the booth but upheld the call on the field.

My question: what's the rule on this? Does a planted toe qualify as in bounds, or only a dragged toe?

I know it's possible my view was week, the reception isn't that great in the hotel room and I just don't trust announcers; I'm just wondering what the rule is on this play.