Cisco Certified Network Associate (CCNA) Practice Exam

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For which transport layer protocol would DNS traffic be matched in an ACL?

ICMP
TCP
UDP
RTP

The correct answer is: UDP

DNS (Domain Name System) primarily uses the User Datagram Protocol (UDP) at the transport layer for its operations. This is because UDP is suited for the simple query-response nature of DNS, where speed is critical, and the overhead of establishing a connection (as with TCP) is unnecessary. DNS queries typically involve a single request followed by a single response, making UDP an efficient choice. In most situations, DNS operates over port 53, which is designated for DNS traffic. While TCP may also be used incrementally in situations when DNS responses are too large to fit in a single UDP packet (which can lead to fragmentation), the standard and most common operation relies on UDP. When configuring access control lists (ACLs) to filter DNS traffic, it is important to match UDP because that is the default protocol used for most DNS queries. Understanding this distinction allows network administrators to effectively manage and secure DNS traffic within their networks.