Cycloalkanes: Introduction, Structure and Physical Properties
Because of their symmetry, monosubstituted cycloalkanes exist only as one isomer and therefore do not require atom numbering to describe the position of the substituent.
However, rings with more than one sustituent require position numbering. One substituent is connected to atom 1 and the next atom containing a sustituent is assigned the next lowest number. Rings can also be considered substituents in complex molecules. In those cases, the cycloalkyl group is used as a prefix.
The basic rules for the nomenclature of molecules are summarized on the IUPAC nomenclature page.