work(),systemsthateffectivelyisolateanorganization'sinternalnetworkstructurefroman()network,suchastheINTERNETisbecomingincreasinglypopular.Thesefirewallsystemstypicallyactasapplication-layer(请作答此空)betweennetworks,usuallyofferingcontrolledTELNET,FTP,andSMTPaccess.Withtheemergenceofmoresophisticated()layerprotocolsdesignedtofacilitateglobalinformationdiscovery,thereexistsaneedtoprovideageneral()fortheseprotocolstotransparentlyandsecurelytraverseafirewall.
A.hosts B.routers C.gateways D.offices正确答案C
