Netopia Router PN Series - Manual (Page 241)

Understanding IP Addressing B-5
Note: If you receive an IP address from an ISP, there must be a
mask associated with that IP address. By using the IP address with
the mask you can discover exactly how many IP host addresses you
actually have.
To configure subnets properly, you must also be able to convert
between binary notation and decimal notation.
Example: Using subnets on a Class C IP internet
Suppose that your organization has a total of 25 IP hosts situated
on three different floors of your office building, and that you are in
charge of designing the network that will connect them. You obtain a
Class C network number, 199.14.17.0, since you expect that your
organization will always have fewer than 254 IP hosts. All your IP
hosts will use IP addresses of the form 199.14.17.x, where x
represents the eight bits that can be used for subnet numbers and
individual host numbers.
How many of the final eight bits of the IP address should you
reserve for hosts, and how many should you use for subnet
numbers? The answer depends on how many subnets you expect to
need, and how many hosts you expect to put on each subnet. All 25
of your hosts could certainly coexist on one network that does not
use subnetting. However, you are fortunate enough to have two IP
routers on hand, so you decide to lower traffic levels and simplify
troubleshooting by setting up three subnets, one for each floor. The
following table lists how many subnets and how many hosts you
may have for a Class C network, depending on how many bits you
allocate to the subnet numbers.