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Q&A

Q1:

How large is the subnet ID field on a Class B network with the mask 255.255.0.0?

A1:

Zero bits (no subnet ID field). The mask 255.255.0.0 is the default condition for a Class B network. All 16 mask bits are used for the network ID, and no bits are available for subnetting.

Q2:

A network admin calculates that he'll need 21 mask bits for his network. What subnet mask should he use?

A2:

21 mask bits: 11111111111111111111100000000000 is equivalent to two full octets plus an additional 5 bits. Each full octet is expressed in the mask as 255. The five bits in the third octet are equivalent to 128+64+32+16+8 = 248. The mask is 255.255.248.0.

Q3:

You have a Class C network address. You also have employees at 10 locations, and each location has no more than 12 people. What subnet mask or masks would enable you to install a workstation for each user?

A3:

255.255.255.240

Q4:

Billy wants to use three subnet bits for subnetting on a Class A network. What should he use for a subnet mask?

A4:

A Class A network means that the first octet will be devoted to the network ID. The first octet of the mask is equivalent to 255. The three subnet bits in the second octet are equivalent to: 128+64+32 = 224. The subnet mask is 255.224.0.0.

Q5:

What IP addresses are assigned in the CIDR range 212.100.192.0/20?

A5:

The /20 supernet parameter specifies that 20 of the IP address will be constant and the rest will vary. The binary version of the initial address is

11010100.01100100.11000000.00000000

The first 20 bits of the highest address must be the same as the initial address, and the rest of the address bits can vary. Show the varying bits as the opposite end of the range (all ones instead of all zeros):

11010100.01100100.11001111.11111111

The address range is 212.100.192.0 to 212.100.207.255.

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