Endohydrolysis of the di-N-acetylchitobiosyl unit in high-mannose glycopeptides and glycoproteins. Does not hydrolyze complex bi- or triantennary glycans. The presence of a core-bound fucose impedes endo F1 hydrolysis. Endohydrolysis of the N,N'-diacetylchitobiosyl unit in high-mannose glycopeptides and glycoproteins containing the -[Man(GlcNAc)2]Asn- structure. One N-acetyl-D-glucosamine residue remains attached to the protein, the rest of the oligosaccharide is released intact. Monomer. Belongs to the glycosyl hydrolase 18 family.