addr: only assign broadcast when subnet over /31

[devsnek]

This mirrors how iproute2 and the kernel handle point to point links.

[gemini-code-assist]

Summary of Changes

Hello @devsnek, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the IPv4 address configuration process by modifying when a broadcast address is explicitly assigned. The change ensures that the system's behavior for broadcast address handling, particularly for point-to-point links (like /31 subnets) and single host addresses (/32 subnets), mirrors the established conventions found in the Linux kernel and iproute2 utility. This improves consistency and adherence to networking standards.

Highlights

• Broadcast Address Assignment Logic: The logic for assigning IPv4 broadcast addresses has been updated to align with standard kernel and iproute2 behavior for point-to-point links.
• Prefix Length Condition: Broadcast addresses are now only explicitly assigned for subnets with a prefix length less than /31. This means /31 (point-to-point) and /32 (single host) subnets will no longer have a broadcast address assigned in this specific code path.

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[gemini-code-assist]

Code Review

This pull request correctly adjusts the logic for setting a broadcast address for IPv4 addresses. The change ensures that a broadcast address is only assigned for subnets with a prefix length smaller than 31 (i.e., for /30 and larger subnets). This aligns with standard networking practices (like RFC 3021) where /31 and /32 prefixes do not have broadcast addresses. The change fixes a bug where a broadcast address was incorrectly calculated for /31 networks and is a valuable improvement. The implementation is correct and no issues were found.