ID
0-255
5
.
.
. 168
64
192
1
3
1
1
5
168
77
192
.
.
.
255
255
256
–224
32
224
255
.
.
.
Copy
Copy
Magic
Copy
Action
IP
64
96
32
0
128 160 192 224
Multiples:
Figure 14-10
Resident Subnet for 192.168.5.77, 255.255.255.224
Resident Subnet Practice Problems
Before moving to the next section, take some time to fill in the blanks in Table 14-10. Check
your answers against Table 14-16 in the section “Answers to Earlier Practice Problems,” later
in this chapter. Complete the table by listing the subnet ID in each case. The text following
Table 14-16 also lists explanations for each problem.
Table 14-10 Practice Problems: Find Subnet ID, Difficult Masks
Problem
IP Address
Mask
Subnet ID
1
10.77.55.3
255.248.0.0
2
172.30.99.4
255.255.192.0
3
192.168.6.54 255.255.255.252
4
10.77.3.14
255.255.128.0
5
172.22.55.77 255.255.254.0
6
1.99.53.76
255.255.255.248
Finding the Subnet Broadcast Address: Difficult Masks
To find a subnet’s broadcast address, a similar process can be used. For simplicity, this pro-
cess begins with the subnet ID, rather than the IP address. If you happen to start with an IP
address instead, use the processes in this chapter to first find the subnet ID, and then use the
following process to find the subnet broadcast address for that same subnet. For each octet:
Step 1.
If the mask octet = 255, copy the subnet ID.
Step 2.
If the mask octet = 0, write 255.
Step 3.
If the mask is neither, identify this octet as the interesting octet:
A.
Calculate the magic number as 256 – mask.
B.
Take the subnet ID’s value, add the magic number, and subtract 1 (ID +
magic – 1).
Technet24
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14
Chapter 14: Analyzing Existing Subnets 337
As with the similar process used to find the subnet ID, you have several options for how to
best learn and internalize the process. If you can, stop reading now, use the companion web-
site for this book, and watch the videos listed for this chapter. Also, look at the examples in
this section, which show the process being used on paper. Then, follow the practice opportu-
nities outlined in the section “Additional Practice for This Chapter’s Processes.”
Subnet Broadcast Example 1
The first example continues the first example from the section “Finding the Subnet ID:
Difficult Masks,” earlier in this chapter, as demonstrated in Figure 14-9. That example started
with the IP address/mask of 130.4.102.1, 255.255.240.0, and showed how to find subnet ID
130.4.96.0. Figure 14-11 now begins with that subnet ID and the same mask.
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