Control: The information systems that collectively provide name / address resolution service for an organisation are fault-tolerant and implement internal / external role separation.
Architecture and Provisioning for Name/Address Resolution Service Supplemental Guidance:
Information systems that provide name and address resolution services include, for example, domain name system (DNS) servers. To eliminate single points of failure and to enhance redundancy, organisations employ at least two authoritative domain name system servers, one configured as the primary server and the other configured as the secondary server. Additionally, organisations typically deploy the servers in two geographically separated network subnetworks (i.e., not located in the same physical facility). For role separation, DNS servers with internal roles only process name and address resolution requests from within organisations (i.e., from internal clients). DNS servers with external roles only process name and address resolution information requests from clients external to organisations (i.e., on external networks including the Internet). Organisations specify clients that can access authoritative DNS servers in particular roles (e.g., by address ranges, explicit lists).