Network address translation (NAT) and Port address translation (PAT) are often used to translate between global and private ip addresses. Private address for Class B is 172.16.0.0 to 172.31.255.255. Only 172.23.0.0/16 fit the range.
Private IP addresses can be drawn from one of the pools of addresses defined in
RFC 1918 as non-routable over the Internet:
• 10.0.0.0 to 10.255.255.255 (Class A private address range).
• 172.16.0.0 to 172.31.255.255 (Class B private address range). The answer is in this range.
• 192.168.0.0 to 192.168.255.255 (Class C private address range).
NAT and PAT translate private IPs to public IPs, so we're seeking a private IP.
Addresses 172.16.0.0 - 172.31.255.255 are private, excluding options A and E.
Using CIDR, consider the remaining answers:
Answer B) 172.18.0.0/10 has subnet mask 255.192.0.0 (10 network bits).
To find network address we need to do a bitwise "AND" operation with the IP and subnet mask:
Comparing bits side by side. If both bits are 1 it's a 1. If any bit is a 0 it’s a 0.
Subnet mask: 11111111.11000000.00000000.00000000(255.192.0.0)
IP address: 10101100.00010010.00000000.00000000(172.18.0.0)
Result: 10101100.00000000.00000000.00000000 (172.0.0.0)
Network: 172.0.0.0
Range: 172.0.0.1-172.63.255.254
Broadcast: 172.63.255.255
Answer D) Repeat for this address.
Both answers are beyond private range.
Answer C is the only one left but you can confirm it with above methods.
GPT gets this one wrong. Here is how I find the correct answer:
(A) 172.15.0.0/18 - wrong, it is a public IP
(B) 172.18.0.0/10 - private IP (good) --> but subnet doesn't offer the most available IP addresses (4 million+)
(C) 172.23.0.0/16 - private IP (good) --> but subnet doesn't offer the most available IP addresses (65 thousand)
**(D) 172.28.0.0/8 - private IP (good) --> AND subnet offers the MOST available IP addresses (16 million+) = best answer
(E) 172.32.0.0/14 - wrong, it is a public IP
So it looks like the best answer is (D)
PS. I will check with my boss who is a Developer and update this answer if it is wrong.
C. 172.23.0.0/16
Network Address Translation (NAT) with Port Address Translation (PAT) is used to translate the private IP addresses of hosts on a LAN to a single public IP address assigned to the firewall. The LAN interface of the firewall typically uses a private IP address range, such as those defined by RFC 1918, that is not globally routable.
172.15.0.0/18, 172.18.0.0/10, 172.28.0.0/8, and 172.32.0.0/14 are all valid private IP address ranges, but 172.23.0.0/16 is commonly used for LAN interfaces in NAT/PAT configurations.
BCD are only IPs where the first 2 octets fall in Class B Private IP range mentioned by Lu5ck. Of those, CIDR 16 keeps the subnet 255.255.0.0 which keeps 172.23 the same. CIDR 10 and CIDR 8 would change the (23 octet) value to fall outside the private IP range for class B.
Adding on to Lu5ck and cyberninja, the reasoning is not just about the classes, but the octets. It is 172.16.0.0/12 to 172.31.0.0/12 range that is RFC 1918. If it is a /8 or /10 it is not in that range. But a 172.16.0.0/12 range can be further sub-netted by using more bits for subnets. So a 172.28.0.0/16 is also in that range
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