How do :default => 0 and :null => false differ for integer fields in migrations?

If I use a migration to update a database, and I add an integer field like this:

t.integer :foo :default => 0, :null => false

What is the default state of existing and new records in the database? I hoping the answer is: - Both will read back foo as 0.

Is default => 0 necessary, if I have :null => false?

Just trying to understand the difference between the two...