On a traditional IPv4 point-to-point WAN link, which subnet mask is typically used to minimize wasted IP addresses while still providing two usable host addresses?

Introduction / Context:
This question focuses on efficient subnetting for point-to-point Wide Area Network links. On such links you typically need only two usable addresses, one for each end of the link. Using a subnet that is larger than necessary wastes addresses. The question reflects the traditional design approach used in many certification exams and production networks.

Given Data / Assumptions:
- The medium is a point-to-point IPv4 WAN link between two routers.
- Only two usable host addresses are required for the link endpoints.
- We want to reduce wastage of IP addresses while remaining compatible with common router configurations and older textbooks.
- Modern /31 usage is known but is not assumed as the default in many exam oriented questions.

Concept / Approach:
In IPv4, a subnet must have enough addresses for the required hosts plus two reserved addresses for the network and broadcast in traditional designs. For a link that requires only two hosts, the smallest traditional subnet that supports this is a /30 network. A /30 has 4 total addresses: one network address, two usable host addresses, and one broadcast address. A /31 has only 2 addresses and is usable on some equipment under specific conditions, but historically and in many exam contexts /30 is the expected answer.

Step-by-Step Solution:
Step 1: Recall the basic subnet formula that total addresses per subnet equal 2^(number of host bits).Step 2: Recognize that a /30 prefix means 32 - 30 = 2 host bits, giving 2^2 = 4 total addresses in the subnet.Step 3: Subtract 2 reserved addresses (network and broadcast) from the 4 total addresses, leaving 2 usable host addresses, which is exactly what a point-to-point link needs.Step 4: Compare with /29, which gives 8 addresses and 6 usable hosts, more than required, and /28, which wastes even more.Step 5: Understand that /31 can be used on some routers but historically was not the default and is often not considered in basic exam questions, so /30 is selected.

Verification / Alternative check:
You can verify by writing a simple example such as 192.0.2.0/30. The addresses in this subnet are 192.0.2.0 (network), 192.0.2.1 (host), 192.0.2.2 (host), and 192.0.2.3 (broadcast). The two usable addresses 192.0.2.1 and 192.0.2.2 are sufficient for the two ends of a point-to-point link, and no additional hosts are required.

Why Other Options Are Wrong:
A /28 subnet provides 16 total addresses and 14 usable host addresses, which is more than needed and wastes address space. A /29 subnet provides 8 total addresses and 6 usable hosts, which still wastes four host addresses. A /31 subnet can be used for special point-to-point designs under RFC 3021, but many basic configurations and exam questions still focus on /30 as the standard answer. A /32 represents a single host route, not a subnet for two routers to share.

Common Pitfalls:
One common pitfall is to forget to subtract the network and broadcast addresses from the total host count. Another is to jump directly to /29 or /28 because they seem small, without formally calculating the usable hosts. Finally, some learners assume that /31 is always valid for any point-to-point link without checking hardware or exam context, which may lead to incorrect answers in traditional certification questions.

Final Answer:
The typical mask used to minimize wasted addresses on a traditional point-to-point WAN link is /30, which is 255.255.255.252.