Question

17. You wish to subnet your class C network address, 222.45.145.0, to support 28 hosts per subnet. Your resulting subnet mask appears as follows: 255.255.255.224. Will the above subnet mask provide the necessary number of host addresses?

18. Your customer owns a Class B address, 147.80.0.0. They want to subnet this address to provide the greatest possible number of host addresses. Which subnet mask provides the greatest total number of hosts across all subnets?

19. Your customer wants to subnet their Class C network to support a maximum of 24 hosts per subnet. What will be the resulting subnet mask?

20. Due to recent business growth, your employer has leased office space in a new building across town. While transitioning, they wish to provide WAN communications between offices. One of your tasks is to subnet their current Class C network address to support the two offices. What subnet mask will support the two subnets, while providing the maximum number of usable host addresses on both subnets?

Answer 1

17. You wish to subnet your class C network address, 222.45.145.0, to support 28 hosts per subnet. Your resulting subnet mask appears as follows: 255.255.255.224. Will the above subnet mask provide the necessary number of host addresses?

Given subnet mask is 255.255.255.224. The binary representation of subnet mask is 11111111.11111111.11111111.1110000, from this binary representation we can see that Network ID is of 27 bits and Host ID part is of 5 bits.

As the Host ID part is of 5 bits, therefore each subnet can have 2⁵ IP addresses, that is a total of 32 IP addresses. From these 32 IP addresses, 2 IP addresses will be reserved for Subnet ID and Limited Broadcast address. Therefore, 30 IP addresses can be used for the Hosts.

We have a requirement of 28 Hosts per subnet. Therefore, the subnet mask 255.255.255.224, will provide the necessary number of host addresses.

18. Your customer owns a Class B address, 147.80.0.0. They want to subnet this address to provide the greatest possible number of host addresses. Which subnet mask provides the greatest total number of hosts across all subnets?

As the given address space is a Class B address space therefore the number of bits reserved for Host ID part is 16 and the bits reserved for Network ID part is 16.

Now, to obtain maximum number of Host addresses in a subnet, the best possible way is to keep the address space as it is. That is the subnet mask will become 255.255.0.0. Thus, there will be a total of 2¹⁶ IP addresses, that is a total of 65536 IP addresses. From these IP addresses two IP addresses will be reserved. Thus, the remaining IP addresses for the Hosts will be 65534.

Therefore, the subnet mask which provides the greatest total number of hosts across all subnets is 255.255.0.0 that is only one subnet.

19. Your customer wants to subnet their Class C network to support a maximum of 24 hosts per subnet. What will be the resulting subnet mask?

The requirement is of 24 hosts per subnet. But while subnetting a network, the IP addresses that must be there in a subnet must be a power of 2. The nearest number greater than 24 which is a power of 2, is 32, which is 2⁵. Therefore, 5 bits will be there for Host ID part and the remaining 27 bits will be for Network ID part.

Therefore, the resulting subnet mask is 11111111.11111111.11111111.11100000, the dotted decimal representation of the same is 255.255.255.224.

20. Due to recent business growth, your employer has leased office space in a new building across town. While transitioning, they wish to provide WAN communications between offices. One of your tasks is to subnet their current Class C network address to support the two offices. What subnet mask will support the two subnets, while providing the maximum number of usable host addresses on both subnets?

Given address space is of Class C type, that is 24 bits are reserved for Network ID and remaining 8 bits are of Host ID part. As the requirement is of two subnets, therefore, we will reserve an additional bit for Network ID part. Thus, resulting in two subnets in the address space. One subnet starting with bit 0 and the other subnet starting with bit 1 for the last 8 bits which are available for the Host ID part in Class C address space.

Therefore, the subnet mask will be 11111111.11111111.11111111.10000000, the dotted decimal representation of the same is 255.255.255.128.