Before moving to the next chapter, practice until you get the right answer most of the time—but use any tools you want and take all the time you need. Then, you can move on with your reading.
However, before taking the exam, practice until you master the topics in this chapter. Gauging a reasonable speed goal is difficult, because some combinations of network ID and mask can yield hundreds or thousands of subnets, while others can yield a much smaller number. So, for speed, before the exam, you should be able to do the first four subnets, which include the zero subnet, in 45 seconds. Table 19-5 summarizes the key concepts and suggestions for this two-phase approach.