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Networking & DNSLinux + IT series

Quiz: 3 networking questions from IT interviews

Three networking questions that come up in IT interviews again and again. Try to answer before you read on.

Quick reference

Q1. How many usable host addresses does a /26 subnet have?

  • a) 64
  • b) 62
  • c) 30

Answer: b) 62. A /26 has 64 addresses. One is the network address and one is broadcast, so 62 are usable.

Q2. DNS normally uses UDP port 53. When does it use TCP 53 instead?

  • a) Never, DNS is UDP only
  • b) For large answers and zone transfers
  • c) Only for DNS over HTTPS

Answer: b) For large answers and zone transfers. When a reply is too big for UDP, and for zone transfers between servers, DNS uses TCP on the same port.

Q3. What does ARP do on a local network?

  • a) Finds the MAC address for an IP
  • b) Gives out IP addresses
  • c) Turns names into IP addresses

Answer: a) Finds the MAC address for an IP. ARP asks who has 192.168.1.1 and gets the MAC address back. DHCP gives out IPs. DNS resolves names.

Explained

  1. How many usable host addresses does a /26 subnet have?

Answer: \62. A /26 leaves 6 bits for hosts, which is 2 to the power of 6, so 64 addresses. The first is the network address and the last is the broadcast address, so 62 can be given to devices. A quick way to remember: /24 gives 254, and each extra bit halves it. /25 gives 126, /26 gives 62, /27 gives 30.

  1. DNS normally uses UDP port 53. When does it use TCP 53?

Answer: for large responses and zone transfers. When a reply does not fit in a UDP packet, the server marks it as truncated and the client asks again over TCP. Zone transfers between DNS servers always use TCP. This is why a firewall that only allows UDP 53 causes failures that are rare and hard to explain, especially with DNSSEC and big TXT records.

  1. What does ARP do on a local network?

Answer: it finds the MAC address for an IP address. Before a PC can send a frame to 192.168.1.1, it broadcasts "who has 192.168.1.1?" and the owner replies with its MAC address. The result is cached in the ARP table, which ip neigh shows on Linux and arp -a on Windows. Two devices answering for the same IP is how you spot an IP conflict, or ARP spoofing.

Interviews test the basics because real outages are so often caused by them.

In one line

Interviews test the basics. Because outages are usually caused by them.

See your arp table

ip neigh

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