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Networking & DNSVisual series

Quiz: 3 infrastructure questions

Three questions show up in almost every infrastructure interview, and the wrong answer is usually the one that sounds clever.

Quick reference

Q1. A RAID 5 array loses one drive. What is still true?

  • a) It survives: parity is spread
  • b) It is gone: no parity here
  • c) It survives with only two drives

Answer: a) It survives: parity is spread. RAID 5 spreads parity across at least three drives and tolerates one failure; capacity is n-1. A second failure during the rebuild ends the set.

Q2. How many /24 subnets fit inside one /16?

  • a) 16
  • b) 256
  • c) 65,536

Answer: b) 256. A /16 holds 65,536 addresses and a /24 holds 256, so 65,536 / 256 = 256 subnets. Each extra prefix bit doubles the count.

Q3. A live video call drops packets. Which transport fits?

  • a) TCP, because it re-sends loss
  • b) UDP, because late data is useless
  • c) Either; the port decides

Answer: b) UDP, because late data is useless. Video needs low latency, and a re-sent frame arrives too late to show. UDP drops it and moves on; WebRTC uses UDP and adds a jitter buffer and congestion control.

Explained

Parity

Question one: a RAID 5 array loses one drive. It survives, because parity is striped across the set and the missing data is rebuilt from the remaining drives. RAID 5 needs at least three drives, gives the capacity of n-1, and tolerates exactly one failure. A second failure before the rebuild finishes ends the array.

Prefixes

Question two: how many /24s fit in a /16? A /16 holds 65,536 addresses; a /24 holds 256, of which 254 are usable. Divide and the answer is 256 subnets, 65,536 addresses in total. Each extra prefix bit doubles the subnet count and halves the size.

Transport

Question three: a live video call drops packets. UDP is the right default, because a re-sent frame arrives too late to show. TCP would hold the stream while it retransmits, which is worse for a call. WebRTC uses UDP, adds a jitter buffer and its own congestion control.

The honest caveat: RAID 5's one-drive tolerance is a promise about drives, not about rebuild time; on large disks a rebuild can run for days, and RAID 6 or RAID 10 is often the safer answer.

The lesson: the basics are not the beginner questions; they are the ones that decide the outage.

In one line

The basics still catch the outage. Parity, prefixes and ports explain most of the pager.

Check the array

mdadm --detail /dev/md0

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