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Which of the following protocols uses both TCP and UDP?

  • A.FTP
  • B.SMTP
  • C.Telnet
  • D.DNS

Answer: D

Correct answer: DNS

DNS uses UDP port 53 for ordinary queries, because a query and its reply are small and a lost one is simply retried, and TCP port 53 when the response exceeds what a single datagram will carry and for zone transfers between servers. FTP, SMTP and Telnet all use TCP alone.

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