Subnet IP Address Calculation
Question
You have a requirement to create a subnet which will have the ability to host 512 usable addresses.
Which of the below network masks would you use to ensure that the ability to host this many IP addresses is as accurate as possible.
Answers
Explanations
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A. B. C. D.Answer - B.
If you have the subnet mask of /23, you will have exactly 512 hosts; out of this 5 IPs are not available to use, because those are reserved IPs.
So, you will end up with 507 hosts which is not meeting our requirement.
Hence, the correct answer would be option.
B.
The first four IP addresses and the last IP address in each subnet CIDR block are not available for you to use, and cannot be assigned to an instance.
For example, in a subnet with CIDR block 10.0.0.0/24, the following five IP addresses are reserved:
10.0.0.0: Network address.
10.0.0.1: Reserved by AWS for the VPC router.
10.0.0.2: Reserved by AWS.
The IP address of the DNS server is always the base of the VPC network range plus two; however, we also reserve the base of each subnet range plus two.
For VPCs with multiple CIDR blocks, the IP address of the DNS server is located in the primary CIDR.
For more information, see Amazon DNS Server.
10.0.0.3: Reserved by AWS for future use.
10.0.0.255: Network broadcast address.
We do not support broadcast in a VPC, therefore we reserve this address.
Please check the below link to know more about it:
https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.htmlTo determine which network mask to use to create a subnet with the ability to host 512 usable IP addresses, we need to calculate the number of available IP addresses in each subnet mask option.
A subnet mask is a binary number that is used to divide an IP address into network and host portions. The subnet mask is represented using a CIDR notation, which is a number that indicates the number of bits used for the network portion of the address. For example, a /24 subnet mask uses the first 24 bits of an IP address for the network portion and the remaining 8 bits for the host portion.
To calculate the number of available IP addresses in a subnet, we need to subtract two addresses from the total number of addresses in the subnet. The first address is the network address, which is used to identify the network itself, and the last address is the broadcast address, which is used to send messages to all hosts on the network.
Using this formula, we can calculate the number of available IP addresses in each of the subnet mask options:
A. /21 subnet mask: 2^11 - 2 = 2046 usable addresses B. /22 subnet mask: 2^10 - 2 = 1022 usable addresses C. /23 subnet mask: 2^9 - 2 = 510 usable addresses D. /24 subnet mask: 2^8 - 2 = 254 usable addresses
Therefore, the option that would provide the ability to host 512 usable IP addresses as accurately as possible would be option A, which uses a /21 subnet mask. This subnet mask provides 2046 usable addresses, which is the smallest number of available addresses that is greater than 512.
Option B would provide fewer addresses than needed, while options C and D would provide far more addresses than needed, which would result in wasted address space.
