Openai·Software Engineer·Technical Phone Screen·Senior

Senior

May 2026

Summary

OpenAI software engineer interview with a coding round focused on IP address iteration. The problem had a clean surface but the follow-ups pushed into bitwise territory pretty fast.

Questions Asked (1)

Design an iterator over a range of IPv4 addresses. It should support both a start/end pair and CIDR notation as input, expose next() and hasNext() methods, and handle large ranges without blowing up memory.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

Started with the CIDR case and immediately fumbled explaining how to extract the host range from the prefix length.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then design a memory-efficient iterator using integer representation of IP addresses. Explain the conversion between IP string and integer, and how to increment and check bounds without storing all addresses. Discuss handling of CIDR notation and potential pitfalls like overflow and invalid inputs.

Pro tip: Mention that you would use unsigned 32-bit integers to avoid sign issues and that you can precompute the end address for CIDR to simplify the iteration logic. Also, consider thread-safety if the iterator might be used concurrently.

1. Clarify requirements and edge cases

Ask about input formats, expected behavior for invalid inputs, inclusivity of endpoints, and whether the iterator needs to be thread-safe. Confirm that memory efficiency is critical.

2. Design the iterator interface and internal state

Define next() and hasNext() methods. Internally, store the current IP as a 32-bit integer and the end IP as a 32-bit integer. For CIDR, compute the start and end addresses from the prefix.

3. Implement IP conversion and iteration logic

Write helper functions to convert IP string to integer and vice versa. In next(), return the current IP and increment it; in hasNext(), check if current <= end. Handle overflow when incrementing past 255.255.255.255.

4. Handle CIDR notation and edge cases

Parse CIDR to get base IP and prefix length, then compute the network address and broadcast address. Ensure the range is correct and handle cases like /0 and /32. Validate inputs and throw exceptions for invalid formats.

5. Discuss trade-offs and optimizations

Compare integer-based iteration vs. string manipulation. Mention that integer approach is O(1) memory and O(1) per next() call. Consider if lazy evaluation is needed and how to handle very large ranges (e.g., /0) without performance issues.

Key Points to Mention

Use 32-bit unsigned integers to represent IP addresses for efficient arithmetic and comparison.
CIDR notation: compute start and end addresses from the prefix length, ensuring correct network and broadcast addresses.
Memory efficiency: store only current and end addresses, not the entire range.
Edge cases: handle /0 (entire IPv4 space), /32 (single address), and overflow when incrementing past 255.255.255.255.
Input validation: reject invalid IP strings and CIDR prefixes, and decide on inclusivity of endpoints.
Thread-safety: if needed, use synchronization or make the iterator immutable and return a new state on each next().

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.