I have the NIVUK, which is equivalent to the NIV 84'. At the end of
Matthew 17:20 is a little "
a" that, when clicked on, gives me this:
"Some manuscripts you. 21 But this kind does not go out except by prayer and fasting."
~The Holy Bible: New International Version—Anglicised. (1984). (electronic edition.). London: Hodder & Stoughton.
The NASB has v21 right where you would expect it to be in the Biblical text, but it puts brackets around it like this (see below) to indicate that this verse was not found in the earliest manuscripts of Matthew's Gospel (manuscripts that had not yet been discovered when the KJV was translated, if memory serves that is).
21 [“But this kind does not go out except by prayer and fasting.”]
~New American Standard Bible: 1995 update. (1995). (Mt 17:21). La Habra, CA: The Lockman Foundation.
With either of those translations then, I am able to see v21, a verse I would not be able to see if I only had access to the oldest available Greek manuscripts, so I'm glad that they both included it (though I prefer the bracketing that the NASB uses), but I'm also glad that they point out the existing problem for us as well so that we are fully informed
--David
p.s. - I have the more modern NIV also. It also has the same "a" that the NIVUK has, but instead of giving me a popup box with the words of v21, it sends me to Mark 9:29. Still like'n the NASB brackets considerably better!