As 3Com's CTO John was responsible for providing strategic engineering direction and development for new products and technologies. Under John, the CTO organization focused on development of new advanced technologies, including major industry contributions to LAN switching, stackable architectures, boundary routing, 10/100/1000 mbps Ethernet infrastructures, converged networks and, most recently, radical simplicity concepts. John, at an individual engineering level, filed several patents each year and also championed 3Com's IP program and helped evolve it from a portfolio of five patents to one of several thousand. Also, during this entire period, Hart was a member of 3Com's Executive Committee, which is responsible for the day-to-day operations of the company.

Prior to joining 3Com, Hart was vice president of engineering and advanced development at Vitalink Communications Corporation where he played a key role in the invention of bridging (the layer 2 technology used in LAN switching). While at Vitalink, Hart also managed the development of routers and network management products. Prior to Vitalink, Hart worked for Control Data Corporation, South Central Bell and Southern Bell in senior network architecture and product development positions.

Throughout his career, Hart has participated in the development of numerous network standards and has been a voting member of IEEE 802 since 1987.