The tremendous success of Internet services has led to the rapid growth of Warehouse-Scale Computers (WSCs). The networking infrastructure has become one of the most vital components in a datacenter. With the rapid evolving set of workloads and software, evaluating network designs really requires simulating a computer system with three key features: scale, performance, and accuracy. To avoid the high capital cost of hardware prototyping, many designs have only been evaluated with a very small testbed built with off-the-shelf devices, often running unrealistic microbenchmarks or traces collected from an old cluster. Many evaluations assume the workload is static and that computations are only loosely coupled with the very adaptive networking stack. We argue the research community is facing a hardware-software co-evaluation crisis. In this dissertation, we propose a novel cost-efficient evaluation methodology, called Datacenter-in-a-Box at Low cost (DIABLO), which uses Field-Programmable Gate Arrays (FPGAs) and treats datacenters as whole computers with tightly integrated hardware and software. Instead of prototyping everything in FPGAs, we build realistic reconfigurable abstracted performance models at scales of O(10,000) servers. Our server model runs the full Linux operating system and open-source datacenter software stack, including production software such as memcached. It achieves two orders of magnitude simulation speedup over software-based simulators. This speedup enables us to run the full datacenter software stack for O(100) seconds of simulated time. We have built a DIABLO prototype of a 2,000-node simulated cluster with runtime-configurable 10 Gbps interconnect using 6 multi-FPGA BEE3 boards.