Question

someone please explain this to me ...

Objective: Review each item below and determine whether it needs a subnet mask. Note the appropriate subnet mask number(s) and the number of bits required.

1. Class A network address on a local network.

2. Class B network address on a local network with 4,000 hosts.

3. Class C network address on a local network with 254 hosts.

4. Class A address with 6 subnets.

5. Class B address with 126 subnets.

6. Class A network address. Currently, there are 30 subnets that will grow to approximately 65 subnets within the next year. Each subnet will never have more than 50,000 hosts.

7. Using the subnet mask from step 6, how much growth will this subnet mask provide?

8. Class B network address. Currently, there are 14 subnets that may double in size within the next two years. Each subnet will have fewer than 1,500 hosts.

9. Using the subnet mask from step 8, how much growth will this subnet mask provide?

Answer 1

Answer: A subnet is always a 32-bit number, with leading consecutive 1s followed by trailing consecutive 0s.

1. Class A address on a local network , will not need a subnet mask because there are no sub-networks i.e the network is not split into subnets.

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2. Yes we need a subnet mask in this case because we are splitting the network into sub-networks to accommodate 4000 host's in it.

Since it's a class B network, first two octates of it's subnet mask would be all ones. The number of host's in the network are given by the formula 2ⁿ- 2, where n = number of trailing zeros in the subnet mask. Hence 2ⁿ - 2 = 4000, or n = 12 .

Hence it's subnet mask would be

11111111.11111111.11110000.00000000

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3. Here number of hosts is 2ⁿ -2 = 254

hence n = 8 which is the number of traling zeros in the subnet mask. Since it's a class C network, it's first three octates are reserved for network address, hence will be all ones. Hence it's subnet mask in binary will be

11111111.11111111.11111111.00000000

4. Since it's a class A network, first octate of the subnet mask will all be ones. Number of subnets is given by the formula 2^N , where N is the number of bits borrowed by the network part from the host part in a subnet mask. To split the network into 6 sub-networks, we need at least 3 ones, because 2³ = 8 > 6. Hence the subnet mask in binary would be

11111111.11100000.00000000.00000000

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5. Since it's class B address, the first two octates of it's subnet mask would be all ones. The number of ones to be taken from the host part to split it into 126 subnets can be found using the formula

Number of subnets = 2^N , where N number of ones to be borrowed from host part.. Thus 2^N = 126 or N = 7.

Hence the subnet mask in binary would be

11111111.11111111.11111110.00000000

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6. Since it's a class A network, first octates in it's subnet mask would be all ones. Since number of subnets after one year will be 65, hence number of ones to be borrowed from host part = 7. So total leading ones in the subnet mask will be 8+7 = 15. We are left with 32-15 = 17 zeros. Hence the total number of hosts it can support is 2¹⁷ = 131072, which is far more than the maximum number of hosts per subnet, hence will easily accommodate 50,000 hosts per subnet.

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7. The network in 6 will provide growth upto 131072 hosts per subnet.

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8.