Digests the beta-1,4-glycosidic bonds in N-acetylglucosamine (GlcNAc) oligomers (mainly dimers). Hydrolysis of terminal non-reducing N-acetyl-D-hexosamine residues in N-acetyl-beta-D-hexosaminides. Glycan degradation; chitin degradation. Monomer. Belongs to the glycosyl hydrolase 20 family.