in the design and management of a directory service. Replication is used to indicate that the same directory namespace (the same objects) are copied to another directory server for redundancy and throughput reasons; the replicated namespace is governed by the same authority. Distribution is used to indicate that multiple directory servers in different namespaces are interconnected to form a distributed directory service; each namespace can be governed by a different authority. Implementations Directory services were part of an Open Systems Interconnection (OSI) initiative for common network standards and multi-vendor interoperability. During the 1980s, the ITU and ISO created a set of standards for directory services, initially to support the requirements of inter-carrier electronic messaging and network-name lookup. The Lightweight Directory Access Protocol (LDAP) is based on the X.500 directory-information services, using the TCP/IP stack and an X.500 Directory Access Protocol (DAP) string-encoding scheme on the Internet. Systems developed before the X.500 include: Domain Name System (DNS): The first directory service on the Internet, still in use Hesiod: Based on DNS and used at MIT's Project Athena Network Information Service (NIS): Originally Yellow Pages (YP) Sun Microsystems' implementation of a directory service for Unix network environments. It played a role similar to Hesiod. NetInfo: Developed by NeXT during the late 1980s for NEXTSTEP. After its acquisition by Apple, it was released as open source and was the directory service for Mac OS X before it was deprecated for the LDAP-based Open Directory. Support for NetInfo was removed with the release of 10.5 Leopard. Banyan VINES: First scalable directory service NT Domains: Developed by Microsoft to provide directory services for Windows machines before the release of the LDAP-based Active Directory in Windows 2000. Windows Vista continues to support NT Domains after relaxing its minimum authentication protocols. LDAP implementations LDAP/X.500-based implementations include: 389 Directory Server: Free Open Source server implementation by Red Hat, with commercial support by Red Hat and SUSE. Active Directory: Microsoft's directory service for Windows, originating from the X.500 directory, created for use in Exchange Server, first shipped with Windows 2000 Server and supported by successive versions of Windows Apache Directory Server: Directory service, written in Java, supporting LDAP, Kerberos 5 and the Change Password Protocol; LDAPv3 certified Apple Open Directory: Apple's directory server for Mac OS X, available through Mac OS X Server eDirectory: NetIQ's implementation of directory services supports multiple architectures, including Windows, NetWare, Linux and several flavours