# Computer Science - Networking - Discussion

Discussion Forum : Networking - Section 1 (Q.No. 6)

6.

You have a class A network address 10.0.0.0 with 40 subnets, but are required to add 60 new subnets very soon. You would like to still allow for the largest possible number of host IDs per subnet. Which subnet mask should you assign?

Discussion:

24 comments Page 1 of 3.
SUNIL KUMAR said:
9 months ago

To accommodate the current and future subnets while allowing the largest possible number of host IDs per subnet, you need a subnet mask that provides a large number of available IP addresses for each subnet.

In this case, you have a Class A network address 10.0.0.0 and want to create 100 (40 current + 60 new) subnets.

To determine the appropriate subnet mask, you need to find the smallest subnet size that can accommodate the required number of subnets (100 in this case) and also provides a sufficient number of host addresses for each subnet.

The formula to find the number of subnets and hosts per subnet is as follows:

The number of subnets = 2^n.

Number of hosts per subnet = 2^(32-n) - 2.

Where "n" is the number of bits in the subnet mask.

Let's calculate for each option:

255.240.0.0(or/1)

Number of subnets = 2^12 = 4096.

Number of hosts per subnet = 2^(32-12) - 2 = 2^20 - 2 = 1,048,574 (approximately).

255.248.0.0 (or/13)

Number of subnets = 2^13 = 8192,

Number of hosts per subnet = 2^(32-13) - 2 = 2^19 - 2 = 524,286.

255.252.0.0 (or /14),

Number of subnets = 2^14 = 16,384.

Number of hosts per subnet = 2^(32-14) - 2 = 2^18 - 2 = 262,142.

255.254.0.0 (or /15),

Number of subnets = 2^15 = 32,768,

Number of hosts per subnet = 2^(32-15) - 2 = 2^17 - 2 = 131,070.

255.255.255.255 (or /32),

This is not a valid subnet mask for general use, as it represents a single IP address with no available host IDs for subnets.

Among the options, the best choice that allows for the largest possible number of host IDs per subnet while accommodating 100 subnets is option 1: 255.240.0.0.

This will provide approximately 1,048,574 host addresses per subnet, which is the highest number compared to the other options.

In this case, you have a Class A network address 10.0.0.0 and want to create 100 (40 current + 60 new) subnets.

To determine the appropriate subnet mask, you need to find the smallest subnet size that can accommodate the required number of subnets (100 in this case) and also provides a sufficient number of host addresses for each subnet.

The formula to find the number of subnets and hosts per subnet is as follows:

The number of subnets = 2^n.

Number of hosts per subnet = 2^(32-n) - 2.

Where "n" is the number of bits in the subnet mask.

Let's calculate for each option:

255.240.0.0(or/1)

Number of subnets = 2^12 = 4096.

Number of hosts per subnet = 2^(32-12) - 2 = 2^20 - 2 = 1,048,574 (approximately).

255.248.0.0 (or/13)

Number of subnets = 2^13 = 8192,

Number of hosts per subnet = 2^(32-13) - 2 = 2^19 - 2 = 524,286.

255.252.0.0 (or /14),

Number of subnets = 2^14 = 16,384.

Number of hosts per subnet = 2^(32-14) - 2 = 2^18 - 2 = 262,142.

255.254.0.0 (or /15),

Number of subnets = 2^15 = 32,768,

Number of hosts per subnet = 2^(32-15) - 2 = 2^17 - 2 = 131,070.

255.255.255.255 (or /32),

This is not a valid subnet mask for general use, as it represents a single IP address with no available host IDs for subnets.

Among the options, the best choice that allows for the largest possible number of host IDs per subnet while accommodating 100 subnets is option 1: 255.240.0.0.

This will provide approximately 1,048,574 host addresses per subnet, which is the highest number compared to the other options.

(2)

Tamene yalew said:
1 year ago

Thanks all for the explanation.

(1)

Shahanaj said:
7 years ago

40 subnets + 60 subnets = 100 subnets required.

2^7= 128 which is nearer to 100 subnets.

We know : 1= 128

2= 192

3= 224

4= 240

5= 248

6= 252

7= 254

8= 256

So, Subnet mask will be : 255. 254.0.0.

2^7= 128 which is nearer to 100 subnets.

We know : 1= 128

2= 192

3= 224

4= 240

5= 248

6= 252

7= 254

8= 256

So, Subnet mask will be : 255. 254.0.0.

(7)

Raksha said:
7 years ago

@Sense.

Thank you.

Thank you.

Sumana said:
7 years ago

Thanks for the explanation.

Lekha said:
7 years ago

(The no of host per subnet are 2^17 - 2 = 131070 (no of 0's are 17))

Can you explain this?

Can you explain this?

GEREMEW BELACHEW said:
7 years ago

Very good. Thanks for all your explanation of the answer.

Sense said:
7 years ago

2^n = Number of Subnets where n is the number of masked on (1) bits in the subnet mask

now a number of Subnets are 60 + 40 = 100.

So, 2^n = 100, solving for n = log 100 /log 2

n=6.6 approximately n = 7

Thus Since the network address is in Class A, so the subnet bits will be in The 2nd eight bits of IP address with 7on bits and 1off bits.

The 1st eight bits will be all on.i.e 11111111 or 255, the 2nd eight bits will be 111111110 or 254 the rest will be 0's.

So the answer will be 255.254.0.0 subnet mask or D.

now a number of Subnets are 60 + 40 = 100.

So, 2^n = 100, solving for n = log 100 /log 2

n=6.6 approximately n = 7

Thus Since the network address is in Class A, so the subnet bits will be in The 2nd eight bits of IP address with 7on bits and 1off bits.

The 1st eight bits will be all on.i.e 11111111 or 255, the 2nd eight bits will be 111111110 or 254 the rest will be 0's.

So the answer will be 255.254.0.0 subnet mask or D.

(1)

Sanjeevini said:
8 years ago

Can anyone help me how to solve this problem?

Harish said:
8 years ago

First of all, we should know what is subnetting -it is a process in which we decide the subnet mask into parts and for that purpose, we have to convert last bit of network(1's) id in to host(0"s) id.

11111111 11111111 00000000 00000000

After converting.

11111111 11111110 00000000 00000000

11111111 11111111 00000000 00000000

After converting.

11111111 11111110 00000000 00000000

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