Most IT interviews test the same core of fundamentals. You don't need to memorize a hundred answers — you need to explain a couple dozen clearly. Here are the ones that come up again and again, with the answer an interviewer wants to hear.
Fundamentals
What happens when you type a URL and press enter?
DNS resolves the domain to an IP, a TCP connection opens (often over TLS on 443), the browser sends an HTTP request, the server responds, and the browser renders the result. Naming the DNS → TCP → TLS → HTTP order is what they're checking.
Difference between TCP and UDP?
TCP is connection-oriented and reliable — it guarantees order and delivery (web, email, SSH). UDP is fire-and-forget and fast, with no delivery guarantee (DNS lookups, video, VoIP).
What's the difference between an IP address and a MAC address?
The IP is a logical address that routes across networks and can change; the MAC is a hardware address burned into the network card, used on the local segment.
Troubleshooting
A user can't reach a website. How do you troubleshoot?
Work the layers: is it just them or everyone? Can they ping the gateway? An external IP? Does ping 8.8.8.8 work but the domain fail (that's DNS)? Check ipconfig/ip a for a valid address. Narrow from local to remote.
What's DNS and why does it break so much?
DNS maps names to IPs. It "breaks" often because it's cached at many layers — a stale or wrong record can linger. "It's always DNS" is a joke because it frequently is.
Security
Explain the CIA triad.
Confidentiality (only the right people see data), Integrity (data isn't tampered with), Availability (systems are up when needed). It's the frame for reasoning about any security control.
Symmetric vs. asymmetric encryption?
Symmetric uses one shared key — fast, but you must share it safely. Asymmetric uses a public/private key pair — slower, but solves key exchange. TLS uses asymmetric to swap a symmetric key, then symmetric for speed.
What is the principle of least privilege?
Give every user and process the minimum access needed to do its job — nothing more. It shrinks the blast radius when something is compromised.
Walk in with every answer ready
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