Information Technology Reference
In-Depth Information
The valid subnets, hosts, and broadcasts are as follows:
Subnet
Hosts
Broadcast
0
1-30
31
32
33-62
63
64
65-94
95
96
97-126
127
128
129-158
159
160
161-190
191
192
193-222
223
224
225-254
255
You can add one more bit to the subnet mask just for fun. You were using 3 bits, which
gave you 224. By adding the next bit, the mask now becomes 240 (11110000).
By using 4 bits for the subnet mask, you get 14 subnets because 2
4
= 16. This subnet
mask also gives you only 4 bits for the host addresses, or 2
4
- 2 = 14 hosts per subnet. As
you can see, the number of hosts per subnet gets reduced rather quickly for each host bit
that gets reallocated for subnet use.
The first valid subnet for subnet 240 is 0, as always. Because 256 - 240 = 16, your
remaining subnets are then 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224,
and 240. Remember that the actual interesting octet value also represents the last valid
subnet, so 240 is the last valid subnet number. The valid hosts are the numbers between the
subnets, except for the numbers that are all 1s—the broadcast address for the subnet.
Table 8.6 shows the numbers in the interesting (fourth) octet for a Class C network with
eight subnets.
Table 8.6
Fourth octet addresses for a Class C network with eight subnets
Subnet
Hosts
Broadcast
0
1-14
15
16
17-30
31
32
33-46
47
48
49-62
63
64
65-78
79
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