As networks approach gigabit performance, supercomputer host interfaces are becaming the communication bottleneck. The Netwrok interface unit (NIU) is a highperformance host interface for pixel planes 5, a custom grapics supercomputer. The design offers both performance and programmability through a balance of data paper, we describe the nIU hardaware and firmware archiecture. We present some preliminary performance measurements and assess the applicabilitly of theis design for the targeted enfironment. We also comment on ist suitability for other environements and outline plans for the future works.