Networks, Hosts, and Subnets for Figure 2.2

TABLE 2 . 6 Networks, Hosts, and Subnets for Figure 2.2 (continued)
3rd Octet
128 64 32 16 8 4 2 1
4th Octet
128 64 32 16 8 4 2 1
Decimal IP Address
(Last 16 bits in bold)
first IP in range 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 172.16.0.1
Last IP in range 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 172.16.0.2
Broadcast 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 172.16.0.3

By simply subtracting the non-0, non-255 (interesting) octet value in our mask (252, in this case,
and there will be only one such value, in any case—all other octet values will be 0 or 255) from 256
(call 256 “the magic number”), we get the increment size that takes us from one subnet boundary
to the next in the octet that we found interesting. Always start with 0 in the interesting octet; that
will be the first valid subnet and will look exactly like the network we started subnetting from.
The next subnet can always be determined by adding the increment to the current value,
yielding 4 in the fourth octet, for the second subnet. This continues until the sum equals the original
value of the interesting octet of the subnet mask (252, in this case), and no octets to the left
of the interesting octet are within your authority to make any larger. Practicing with this shortcut
method is the only way to truly understand each of these nuances.
After picking the first available network number (172.16.0.0), given our 30-bit subnet mask
and eliminating host IP addresses that are all 1s and all 0s, we have the following range of numbers:
172.16.0.1–172.16.0.2. The broadcast address is all host bits on, or 172.16.0.3, which is
also the address right before the next subnet address. We can take one of these numbers and assign
it to one side of the serial link. The other number can be assigned to the other end of the serial link.
Next, we will calculate the range of IP addresses to use for our switched subnet, containing four
router interfaces. We pick the first available network address, given our 29-bit subnet mask and
previous choices. In this case, the first available network is 172.16.0.8, as shown in Table 2.7.