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Subnet

0.0

64.0

128.0

192.0

First host

0.1

64.1

128.1

192.1

Last host

63.254

127.254

191.254

255.254

63.255

127.255

191.255

255.255

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 mask

• Subnets? 24 = 16.

• Hosts? 212 – 2 = 4094.

• Valid subnets? 256 – 240 = 0, 16, 32, 48, etc., up to 240. Notice that these are the same

numbers as a Class C 240 mask.

• Valid hosts?

The following table shows the first four subnets, valid hosts, and broadcast addresses in a Class B 255.255.240.0 mask:

Subnets? 22 = 4. Hosts? 214 – 2 = 16,382 (6 bits in the third octet, and 8 in the fourth).

Valid subnets? 256 – 192 = 64. 0, 64, 128, 192. Remember the subnetting is performed in the third octet, so the subnet numbers are really 0.0, 64.0, 128.0, and 192.0, as shown in the next table.

123

• Valid hosts?

The following table shows the four subnets available, the valid host range, and the broadcast address of each:

Subnet

0.0

16.0

32.0

48.0

First host

0.1

16.1

32.1

48.1

Last host

15.254

31.254

47.254

63.254