Undergraduate and Graduate level course numbering is as such:

001-099 Pre-college level courses (not applicable to a degree).
100-299 Lower-division courses (when applicable, 100-199 freshmen; 200-299 sophomores) undergraduate credit only.
300-499 Upper-division courses (when applicable, 300-399 junior; 400-499 senior) undergraduate credit only.
500-599 Advanced upper-division courses in which graduate students may enroll and receive graduate credit. (It is assumed that graduate students will perform more requirements and be graded more strictly than undergraduates in these courses.)
600-799 Graduate courses (master's and doctoral level).
800-899 Graduate courses (limited to doctoral candidates).
900-999 Post-doctoral (or post-terminal) degree courses only.