In terms of flaws, the PUP protocol family was not device independent, in modern terminology the IP and MAC layers were combined into a single layer, which made wide scale adoption difficult. PUP's 8-bit network and 8-bit host could scale to at most 64k machines, before an inter-network bridge or gateway would be needed. For this reason a successor, the XNS (Xerox Networking System) was developed by the Xerox Office Systems Division using many of the ideas of PUP, but also including a globally unique 48-bit host identifier (which became MAC address in DIX v2 and later IEEE 802.3) which solved these problems: