Does every router support /31 point-to-point addressing?

Most modern routing equipment supports RFC 3021 /31 addressing, but older or more constrained devices may have incomplete support — checking the specific equipment's documentation before standardizing on /31 addressing is worth doing, particularly in a mixed-vintage equipment environment.

Can /31 addressing be used for anything besides point-to-point links?

RFC 3021 specifically scopes this exception to point-to-point links — applying a /31 to a subnet with more than two devices (or any shared-medium/broadcast-domain subnet) doesn't make sense, since the whole rationale for the exception (no meaningful broadcast use case) depends on the link genuinely connecting exactly two endpoints.

Is a /30 subnet's "wasted" 2 addresses (network and broadcast) actually a real cost?

For a network with only a handful of point-to-point links, the address-space cost of using /30 instead of /31 everywhere is genuinely negligible. It becomes a more meaningful consideration specifically for large networks with many point-to-point links (large WANs, service provider backbones), where the cumulative savings across dozens or hundreds of links can be a real factor in overall address-space planning.

Engineering·5 min read·August 14, 2026

🔗 /30 vs. /31 for Point-to-Point Links: The RFC 3021 Exception

Why point-to-point router links traditionally use a /30 subnet, how the RFC 3021 /31 exception eliminates the usual network/broadcast address reservation, and when each is appropriate.

By EngineersUniverse Editorial Team

Engineering Editorial & Technical Review Team

Published August 14, 2026

Why Point-to-Point Links Have a Distinct Sizing Need

A point-to-point link — a direct connection between exactly two routers, with no other devices sharing that link — has a fixed, minimal addressing requirement: exactly 2 usable host addresses, one for each end of the link. This is different from a typical LAN subnet, where host count depends on how many devices will eventually connect and typically includes room for growth — a point-to-point link's host count is fixed by definition at exactly 2, since a third device simply can't be added to a genuine point-to-point connection.

The Traditional /30 Approach

Applying standard subnetting logic, the smallest subnet that provides at least 2 usable host addresses is a /30 — 4 total addresses, with 2 reserved (network and broadcast) and exactly 2 usable, a perfect match for a point-to-point link's fixed requirement. This has been, and remains, the traditional and still very widely used approach for addressing point-to-point links, and it works cleanly within standard subnetting rules without requiring any special exception.

The RFC 3021 /31 Exception

RFC 3021 (published in 2000) defines a specific exception to the usual "reserve the network and broadcast address" rule for the special case of a /31 subnet used on a point-to-point link: instead of reserving 2 of the 4... wait, a /31 only has 2 total addresses to begin with (1 host bit, 2^1 = 2 total addresses) — RFC 3021 specifies that for this specific point-to-point use case, both addresses in a /31 subnet can be used as host addresses, with neither treated as a reserved network or broadcast address. This effectively doubles addressing efficiency for point-to-point links compared to the traditional /30 approach, using only 2 addresses total instead of 4.

Why This Exception Exists Specifically for Point-to-Point Links

The normal justification for reserving a network and broadcast address doesn't meaningfully apply to a true point-to-point link — broadcast addressing is primarily useful on a shared-medium subnet with multiple potential recipients, and a link with exactly two directly connected endpoints has no meaningful use for a broadcast address distinct from simply addressing the other end directly. RFC 3021 formalizes this observation into a standard, interoperable exception specifically scoped to point-to-point links, rather than proposing to eliminate network/broadcast reservation for subnetting generally (where it remains meaningful and necessary for normal multi-host subnets).

Why /30 Remains Common Despite /31 Being More Efficient

Even though /31 addressing is more address-space-efficient and is well-supported by modern routing equipment, /30 addressing for point-to-point links remains extremely common in practice — partly from organizational convention and existing documentation/configuration standards that predate widespread /31 adoption, partly because some older or more constrained network equipment may have incomplete or inconsistent /31 support, and partly because the address-space savings of /31 vs. /30 (2 addresses saved per link) is genuinely marginal for many networks that aren't under severe address-space pressure. Organizations with large numbers of point-to-point links (particularly large service-provider or enterprise WAN environments) are more likely to have specifically adopted /31 addressing to capture the cumulative address-space savings across many links.

Practical Guidance for Choosing Between Them

For a new deployment where address space efficiency genuinely matters (a large network with many point-to-point links, or an organization managing a constrained private address allocation), /31 addressing per RFC 3021 is the more efficient, standards-compliant choice, assuming the network equipment supports it. For smaller networks, environments with legacy equipment of uncertain /31 support, or organizations simply following established internal addressing conventions, /30 remains a perfectly valid, universally supported choice — this is a case where "more efficient" doesn't automatically mean "the answer that fits every situation."

Topics covered

point to point subnet /30RFC 3021 /31router link subnet sizingpoint to point IP addressing