正解:D
Explanation
A /30 subnet is the smallest possible subnet that can be used for a point-to-point configuration between two routers. A /30 subnet has only two usable host addresses, one for each router, and a network address and a broadcast address. A /30 subnet conserves IP space by minimizing the number of wasted addresses. A /24, /26, or /28 subnet would have more usable host addresses than needed for a point-to-point configuration and would waste IP space.
References:
Routing Technologies - N10-008 CompTIA Network+ : 2.21
CompTIA Network+ Certification Exam Objectives, page 10