Which one of the following protocols uses both UDP and TCP ports for the transport layer operation?

A．FTP

B．TFTP

C．SMTP

D．Telnet

E．DNS

参考答案

正确答案：E
解析：Explanation:TCPandUDPmustuseportnumberstocommunicatewiththeupperlayers.Portnumberskeeptrackofdifferentconversationscrossingthenetworksimultaneously.Originating-sourceportnumbersaredynamicallyassignedbythesourcehost,whichwillbesomenumberstartingat1024.1023andbelowaredefinedinRFC1700,whichdiscusseswhatiscalledwell-knownportnumbers.Virtualcircuitsthatdonotuseanapplicationwithawell-knownportnumberareassignedportnumbersrandomlychosenfromwithinaspecificrangeinstead.TheseportnumbersidentifythesourceanddestinationhostintheTCPsegment.OnlyDNSusesbothUDPandTCPport53.