The domain is all the x's on a graph or that are permitted in an equation (if you have an equation instead of a graph)
If there is nothing on the graph to indicate the graph is a segment (endpoints that are either closed or open) or has other holes in it (literally a point on the line that is open) then the domain for a linear function (a line) is always all real numbers. The same is true for all of the above for the range as well, except the range is all the y's
