VLSM design for point-to-point WAN links On a Variable Length Subnet Mask (VLSM) IPv4 network, which subnet mask should be assigned to point-to-point serial WAN links to minimize wasted host addresses while remaining standards-compliant?

Introduction / Context:
When you design an IPv4 network using Variable Length Subnet Masking (VLSM), you want to allocate just enough addresses for each segment. Point-to-point WAN links connect exactly two endpoints, so they need the smallest subnet that still supports two usable host addresses and standard routing behavior. Choosing the right mask reduces address waste and simplifies documentation.

Given Data / Assumptions:

• IPv4 addressing using VLSM is permitted.
• The link is a classic point-to-point WAN segment that must support exactly two IP nodes.
• Conventional routing and legacy devices are in scope (that is, we assume two usable host addresses plus network and broadcast).

Concept / Approach:

Each IPv4 subnet historically reserves two addresses: the network ID and the directed broadcast. A /30 network (mask 255.255.255.252) has 4 total addresses: 1 network, 2 hosts, 1 broadcast. This perfectly fits a two-endpoint link. While /31 (255.255.255.254) can be used on many modern routers for point-to-point links, traditional interoperability and some exam objectives assume /30 for two usable hosts plus broadcast semantics.

Step-by-Step Solution:

Verification / Alternative check:

Calculate usable hosts by formula: usable_hosts = 2^(32 - prefix) - 2. For /30, usable_hosts = 2^(2) - 2 = 2. This matches the requirement for a two-node link. Field deployments confirm /30s are standard on serial links across vendors.

Why Other Options Are Wrong:

• /27, /28, /29: These provide 30, 14, and 6 usable hosts respectively, which wastes addresses on a two-host link.
• /31: Although valid on many modern platforms for point-to-point, many curricula and mixed-vendor networks still prefer /30. The question emphasizes minimizing waste while staying broadly compatible.

Common Pitfalls:

• Forgetting that legacy gear might not support /31 semantics.
• Accidentally assigning a /29 and later discovering unused hosts across many links.

Final Answer:

/30