Netopia Router PN Series - Manual (Page 243)

Understanding IP Addressing B-7
that allocates five bits to the host address and three to the subnet
address. This gives you a potential of six subnets of 30 machines
each.
2. Determine the subnet mask
You can find the subnet mask associated with your subnetting
choice in the table above. IP does not specify which bits are to be
used for the subnet numbers and which for the host numbers, but it
is conventional to use the left-most bits for the subnet numbers.
This allows you to use an unbroken series of host numbers on each
subnet, although there will be big gaps between your subnet
numbers.
Now you can calculate the legal range of host numbers you can use
on each of your subnets, the legal subnet numbers you can use,
and the combined totals to use when setting up each host with its
own IP address.
3. Find the actual host numbers
First, determine which host numbers are legal. In this example, host
numbers are five bits long, meaning that values can range from
00000 to 11111 in binary notation. Remember that 00000 and
11111 are reserved for broadcasts. Actual host numbers,
therefore, would be 00001 through 11110 in binary notation, or 1
through 30 in decimal notation.
4. Find the actual subnet numbers
Next, determine which subnet numbers are legal. In this example,
the subnet numbers could be any eight bits that meet two
constraints. The first constraint is that the five least significant
digits must be zero, as these bits are allocated to the host number
and cannot be used in the subnet number. The second constraint is
that the three most significant bits must not be all zeros or all ones,
as these values are reserved for broadcasting. So the legal subnet
numbers are 00100000, 01000000, 01100000, 10000000,
10100000, and 11000000. When translated into decimal notation,
these possible subnet numbers are 32, 64, 96, 128, 160, and
192. You decide to use 32, 64, and 96 as the subnet numbers for
your three subnets: 32 for the third floor, 64 for the fourth floor, and
96 for the fifth floor.