boundary vs extent

boundary and extent both are nouns.

Word	Noun	Adjective	Verb	Adverb
boundary	Yes	No	No	No
extent	Yes	No	No	No

As nouns, extent is a hypernym of boundary; that is, extent is a word with a broader meaning than boundary:

boundary (noun)	extent (noun)
the greatest possible degree of something	the distance or area or volume over which something extends
the line or plane indicating the limit or extent of something	the point or degree to which something extends
a line determining the limits of an area