Subnetting
159
The following table shows the two subnets available, the valid host range,
and the broadcast address of each:
Notice that we just added the fourth octet's lowest and highest values
and came up with the answers. Again, it's pretty much the same as it is for
a Class C subnet--we just added 0 and 255 in the fourth octet.
Practice Example #2B: 255.255.240.0 (/20)
172.16.0.0
= Network address
255.255.240.0
= Subnet address
Subnets? 2
4
- 2 = 14.
Hosts? 2
12
- 2 = 4094.
Valid subnets? 256
- 240 = 16, 32, 48, etc., up to 224. Notice these are
the same numbers as a Class C 240 mask.
Broadcast address for each subnet?
Valid hosts?
The following table shows the first three subnets, valid hosts, and broad-
cast addresses in a Class B 255.255.240.0 mask:
Practice Example #3B: 255.255.254.0 (/23)
Subnets? 2
7
- 2 = 126.
Hosts? 2
9
- 2 = 510.
Valid subnets? 256
- 254 = 2, 4, 6, 8, etc., up to 252.
Subnet
64.0
128.0
First host
64.1
128.1
Last host
127.254
191.254
Broadcast
127.255
191.255
Subnet
16.0
32.0
48.0
...
First host
16.1
32.1
48.1
...
Last host
31.254
47.254
63.254
...
Broadcast
31.255
47.255
63.255
...
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