Openai·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Apr 2026Remote

Summary

Got a coding screen at OpenAI that was basically one problem: build an iterator over all IPs in a CIDR block. Seemed niche at first but the bitwise mask stuff is where they actually wanted to see you think.

Questions Asked (1)

Implement a Python iterator class that yields every IP address in a given CIDR block, such as '192.168.1.0/24'.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The iterator protocol part was fine, __init__ __iter__ __next__, no surprises there.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements: should the iterator yield all addresses including network and broadcast? Then outline the design: parse CIDR, compute network address and total addresses, and implement __iter__ and __next__ to yield each address. Discuss trade-offs like memory efficiency (lazy generation) and edge cases (IPv4 vs IPv6, invalid CIDR).

Pro tip: Mention that you'd use Python's built-in ipaddress module for parsing and validation, but also be prepared to explain how you'd implement the bitwise operations manually if asked to avoid libraries.

1. Clarify requirements and edge cases

Ask whether to include network and broadcast addresses, whether IPv6 support is needed, and how to handle invalid input. This shows attention to detail and avoids incorrect assumptions.

2. Design the iterator class

Outline a class with __init__ that parses the CIDR, stores the current address and end address, and implements __iter__ (returns self) and __next__ (returns current and increments, raising StopIteration when done).

3. Implement address arithmetic

Explain how to convert IP to integer, compute the network address and total number of addresses (2^(32-prefix)), and iterate by incrementing the integer and converting back to IP.

4. Discuss trade-offs and optimizations

Mention memory efficiency of lazy generation, potential performance for large blocks, and whether to use built-in libraries vs manual bit manipulation. Also consider thread-safety if relevant.

5. Test and validate

Walk through test cases: small CIDR (e.g., /30), edge cases like /32 and /0, and invalid inputs. Show how to verify correctness.

Key Points to Mention

Use of Python's ipaddress module for parsing and validation, but also explain manual bitwise operations.
Lazy evaluation to avoid storing all IPs in memory, especially for large blocks.
Handling of network and broadcast addresses (inclusion/exclusion based on requirements).
Edge cases: /32 (single address), /0 (entire IPv4 space), and invalid CIDR strings.
Time and space complexity: O(1) space, O(n) time where n is number of addresses.
Potential for IPv6 support and differences in address size (128 bits vs 32 bits).

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