This thesis investigates distributed router architectures and IP networks with centralized control. While the current trend in IP-router architectures is towards decentralized design, there have also been research proposals for centralizing the control functions in IP networks. With continuous evolution of routers and IP networks, we believe that eventually IP networks in an autonomous system (AS) and a distributed router might converge into one network system. This system, which can be considered both as a distributed router and a centrally-controlled IP network, is divided into a control plane and a forwarding plane. The control plane is responsible for routing, management and signalling protocols, while the forwarding plane is responsible for forwarding packets. The work in this thesis covers both the forwarding and control planes. In the forwarding plane, we study network processor systems that function as forwarding elements in a distributed router. We introduce a system model and a simulation tool based on the model. Using the simulation tool, we investigate network processor system design by studying throughput, utilization, queueing behavior and packet delays. In addition to network processor systems, we study IP-address lookup, which is one of the key packet processing functions in Internet routers. Our work in IP-address lookup contains an efficient lookup algorithm, a scheme to divide the lookup procedure into two-stages in a distributed router, and an approach to perform efficient lookup on a router supporting multiple virtual routers. In the control plane, we study three emerging research issues with centralized control. We provide a thorough study of the routing convergence process in networks with centralized control, and compare it with decentralized link-state routing protocols. We propose an efficient approach to perform traffic engineering and routing in networks with centralized control, and compare it with an approach using optimized link weights. Finally, we present an approach to perform loop-free updates of forwarding tables when the forwarding paths change. This loop-free update approach is particularly useful in networks with centralized control. The results presented in this thesis are useful for building next-generation routers and IP networks with centralized control that might eventually converge into one network system. / QC 20100726