The base charges are generally based upon one of two items. The first of these is the amount of space, commonly referred to as racks, U's or Cabinets which a server will use at the colocation facility.
In order to standardize computer and networking hardware located in machine room facilities, the industry is standardized on a 19" rack design. This is essentially a form of cabinet or stand that has horizontal mounting hardware spaced 19" apart. This allows for all hardware manufacturers to design their equipment to be easily installed inside of the racks.
The overall height of the items in the racks can vary, but the standard height unit or U for a rack mount case is roughly 1.75 inches high. Components are then labeled with their height in terms of the number of U's. A standard industry server size is typically 1U while most networking gear, such as switches, will be 2U and drive upwards of 4U or greater.
The number of U's required to mount the hardware into the colocation provider will determine the base price of the server. A single 1U server will typically be less expensive than a larger 2U system. To use full sized expansion cards found in most desktop computer systems, the server would need to be in a 4U server case which is significantly more expensive. This means that when determining the costs of the server, one must also look at the costs and equipment necessary to fit in that space.
A cabinet is typically the term used to refer to a full sized rack or about 47U in space for equipment mounting. A cabinet that has additional security such as a locking door panel might be referred to as a cage or closed cabinet and provides an additional level of security to the hardware installed in the facility. Cabinets are typically rented out by larger corporations that need a fair amount of space for multiple servers and arrays of storage.
Partial cabinets are also available at most colocation facilities. Typically they are either a 1/2 cabinet (about 24U) or 1/4 cabinet (about 12U). This is still a large amount of space for hardware to be installed and will cost a fair amount of money. The reason for mentioning this is that most providers will give quotes for a 1/4 cabinet if the hardware is not rack mountable. For a single server it can be a huge cost migrating to a dedicated rack server case that is as large as the 4U size.