Databricks·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Databricks SWE interview with a networking-flavored coding problem. Not the typical graph or DP question I was expecting, which threw me off a bit at first.

Questions Asked (1)

Build a function that takes a list of firewall rules (each mapping a CIDR block or exact IP to ALLOW or DENY) and an IP address, then returns the action from the first matching rule. A rule matches if the leading bits of the IP match the network portion defined by the prefix length.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I hadn't touched CIDR notation since a networking class years ago so there was a solid 30 seconds of internal panic.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the input format and matching semantics, then outline a solution that parses each rule into a network address and prefix length, converts the IP to an integer, and checks rules in order using a bitmask. Discuss trade-offs between linear scan and more efficient structures like a trie, and mention edge cases such as IPv6 or invalid input.

Pro tip: Mention that you would normalize the IP and CIDR to integers and use bitwise operations for O(1) matching per rule, and proactively discuss how you'd handle IPv6 or overlapping rules to show depth.

1. Clarify requirements and assumptions

Ask about the rule format (e.g., '192.168.0.0/24', '10.0.0.1'), whether IPv6 is needed, and how to handle invalid inputs or no matching rule. Confirm that rules are evaluated in order and the first match wins.

2. Design the matching logic

Explain that you'll convert the IP and each rule's network address to a 32-bit integer (or 128-bit for IPv6). For a rule with prefix length p, compute a mask with the top p bits set, then check if (ip & mask) == (network & mask).

3. Implement the function

Iterate through the rules in order, parse each rule's CIDR or exact IP, perform the bitwise match, and return the action of the first match. If no rule matches, return a default action (e.g., DENY) or as specified.

4. Analyze complexity and trade-offs

State that the linear approach is O(n) per lookup, which is fine for small rule sets. For large sets, discuss using a trie (prefix tree) for O(prefix length) lookup, or sorting rules by prefix length if longest-prefix match is desired instead of first-match.

5. Test with edge cases

Walk through examples: exact IP match, /32 and /0 rules, overlapping rules, invalid CIDR, and IPv6 if applicable. Verify that the first matching rule is returned and that the mask calculation is correct.

Key Points to Mention

Bitwise operations for efficient matching: convert IP to integer, compute mask from prefix length, and use (ip & mask) == (network & mask).
Handling both CIDR blocks and exact IPs: an exact IP is equivalent to a /32 (or /128 for IPv6) rule.
Order of evaluation: rules are checked sequentially and the first match determines the action.
Edge cases: no matching rule, invalid input, /0 (match all), and IPv6 support.
Trade-offs: linear scan vs. trie for performance, and first-match vs. longest-prefix match semantics.
Complexity analysis: O(n) time per lookup for linear scan, O(1) space; trie offers O(prefix length) lookup but higher space.

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