An engineer must configure a /30 subnet between two routes... (CCNA 200-301)

Correct answer: D) interface e0/0 description to XX-AXXX:XXXXX ip address 209.165.201.2 225.255.255.252.
Let's dive deeper into why option D with the IP address 209.165.201.2 and subnet mask 255.255.255.252 is the correct configuration for a /30 subnet between two routers.

1. Understanding /30 Subnets:
A /30 subnet uses 30 bits for the network portion of the IP address, leaving only 2 bits for host addresses. This translates to a subnet mask of 255.255.255.252 (binary: 11111111.11111111.11111111.11111100).
This creates a very small subnet with only 4 IP addresses:
Network address: The first address in the subnet, not usable for hosts.
Broadcast address: The last address in the subnet, used for broadcasting messages to all devices on the subnet.
Two usable IP addresses: The remaining two addresses in the subnet, available for assignment to devices like routers.

2. Analyzing Option D:
IP address: 209.165.201.2 falls within the range of usable IP addresses for a /30 subnet with network address 209.165.201.0 (calculated by clearing the last two bits of the given IP address).
Subnet mask: 255.255.255.252 correctly identifies the network portion of the IP address and confirms it belongs to a /30 subnet.

3. Visualization:
Here's a visual representation of the /30 subnet with option D highlighted:
!
As you can see, 209.165.201.2 sits comfortably within the range of usable addresses (green), separated from the network and broadcast addresses (red).

4. Advantages of /30 Subnets:
Point-to-point connections like links between two routers often only require two IP addresses. Using a /30 subnet avoids wasting IP addresses compared to larger subnets.
/30 subnets improve traffic isolation and security by creating smaller broadcast domains, limiting the impact of broadcasts on the rest of the network.

5. Conclusion:
Therefore, based on the understanding of /30 subnets, analysis of option D, and its advantages, it's clear that 209.165.201.2 with subnet mask 255.255.255.252 is the optimal configuration for a /30 subnet between two routers.