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IP Addressing and Subnetting 265
For one other example, Table 5-23 shows network 128.1.0.0, with mask 255.255.252.0.
Given a Network Number and a Static Subnet Mask, How Many Hosts per Subnet, and
How Many Subnets?
Finally, one more way the exam is likely to test your knowledge of IP addressing and subnetting
is to ask which subnet masks will meet a set of requirements. This last type of question would
be phrased as something like, "If you need at least 30 hosts in each subnet and only 4 subnets,
and if you are using network 192.1.1.0, which of the following masks meet your requirements?"
To answer these types of questions confidently, you must have a good understanding of the three
parts of an IP address. Figure 5-19 earlier in the chapter shows the network, subnet, and host
parts of an address. These fields and their sizes are important because they identify the number
of subnets possible and the number of hosts per subnet.
The number of hosts per subnet is defined by this formula:
2
number of host bits
­ 2
The number of host bits in an address is equal to the number of binary 0s in the subnet mask.
Table 5-23
Valid Subnet Numbers, 128.1.0.0, Mask 255.255.252.0
Step 1
Interesting byte is 3
Step 2
Increment 2
2
=
4
Step 3
128.1.____.0
128.1.____.0
128.1.____.0
128.1.____.0
128.1.____.0 and so forth
Step 4
128.1.0.0
Step 5
128.1.4.0
128.1.8.0
128.1.12.0
Skipped a few for brevity
128.1.252.0 (This is the last one)
Step 6
128.1.256.0 (This one is invalid)
ch05.fm Page 265 Monday, March 20, 2000 5:06 PM