Hi, Mike. Do you have iron or steel pipe? Steel pipe has a smoother finish and is stencilled A-53 somewhere on it. This is used for gas lines commonly and, if galvanized, for water piping in older applications. Steel pipe must be threaded. You will still see the size shown as IPS "iron pipe size" somewhere in the stencil, but if it is A-53, it is steel pipe. Depending on your level of expertise, this may be a job for a plumber and probably is.
Cast iron pipe is used for sewer lines and is cast with an upset end for one piece to slip into another. This is joined with oakum and lead and this is definately a job for a plumber.
Good luck and don't take a chance with gas lines. Henry in MI