Arista Networks, Inc. ANET

The business

Arista makes the switches that connect servers to each other inside a data centre. When a hyperscale operator wires up tens of thousands of accelerators, the network between them determines how fast the whole cluster runs, and Arista's advantage is a single operating system, EOS, running across every box rather than the patchwork most competitors ship.

The customers are the largest cloud and artificial-intelligence builders in the world, which is why the concentration is what it is. Arista sells to perhaps a few hundred organisations that matter, and a handful of them account for most of the growth. That is a very different business from selling networking equipment to thousands of enterprises, and it explains both the margins and the risk.