A rack (or cabinet) is the standardized 19-inch metal enclosure that holds servers and network gear. Its height is measured in U (rack units): 1U = 44.45 mm; a standard rack is 42U.
In colocation you rent a full rack, half-rack, quarter-rack, or per unit (1U, 2U…). Each rack is fed by power strips (PDUs) and its density is capped by available electrical power (in kVA) and cooling capacity.
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