C18H38, recognized as a string of 18 simply-linked carbon atoms, like so:

  H H H H H H H H H H H H H H H H H H
  | | | | | | | | | | | | | | | | | |
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-H
  | | | | | | | | | | | | | | | | | |
  H H H H H H H H H H H H H H H H H H
Other forms of C18H38 are not considered octadecane, but instead are identified by naming the longest string of carbon atoms, then referencing the branches off of them, as in 2-methyl heptadecane, below:
  H H H H H H H H H H H H H H H H H
  | | | | | | | | | | | | | | | | |
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-H
  | | | | | | | | | | | | | | | | |
  H | H H H H H H H H H H H H H H H
  H-C-H
    |
    H

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