Reducing-end exoglycosidase involved in glycan degradation (PubMed:1527079, PubMed:16115860). Hydrolyzes N,N'-diacetylchitobiose (GlnNAc2) and higher chitin-oligosaccharides (GlcNAc(n)) to monomeric GlcNAc acting on reducing-end residues (PubMed:1527079, PubMed:16115860, PubMed:16794344). During N-linked glycoprotein degradation, it cleaves single GlcNAc molecules uniquely from the reducing end of N,N'-diacetylchitobiose units linking oligosaccharides to proteins, and therefore it functions after cleavage of the Asn- GlcNAc bond and asparagine removal by glycosylasparaginase (By similarity). {ECO:0000250|UniProtKB:Q01460, ECO:0000269|PubMed:1527079, ECO:0000269|PubMed:16115860, ECO:0000269|PubMed:16794344}. This is the function of Ensembl gene identifier ENSG00000117151 (CTBS, chitobiase).