Carbon number 1 is linked to the nitrogenous base (adenine, cytosine, guanine, or thymine).
Carbon number 3 is linked to the next nucleotide by a phosphate group.
Carbon number 5 is linked to the previous nucleotide by a phosphate group.
(The "next" or "previous" nucleotide depend on the direction your seeing it. I considered the 5'->3' direction.)
All the other carbons do not link to any other DNA component.
