Domain controller Server

On Windows Server Systems, a domain controller (DC) is a server that responds to security authentication requests (logging in, checking permissions, etc.) within the Windows Server domain.
In Windows NT, one domain controller per domain was configured as the Primary Domain Controller (PDC); all other domain controllers were Backup Domain Controllers (BDC). A BDC could authenticate the users in a domain, but all updates to the domain (new users, changed passwords, group membership, etc) could only be made via the PDC, which would then propagate these changes to all BDCs in the domain. If the PDC was unavailable (or unable to communicate with the user requesting the change), the update would fail. If the PDC was permanently unavailable (e.g. if the machine failed), an existing BDC could be promoted to PDC. Because of the critical nature of the PDC, best practices dictated that the PDC should be dedicated solely to domain services, and not used for file/print/application services that could slow down or crash the system. Some network administrators took the additional step of having a dedicated BDC online for the express purpose of being available for promotion if the PDC failed.
Windows 2000 and later introduced Active Directory ("AD"), which largely eliminated the concept of primary and backup domain controllers in favor of multi-master replication. However, there are still a number of roles that only one domain controller can perform, called the Flexible single master operation roles (some of these roles must be filled by one DC per domain, while others only require one DC per AD Forest). If the server performing one of these roles is lost the domain can still function, and if the server will not be available again, an administrator can designate an alternate DC to assume the role (a process known as "seizing" the role). This is a procedure used at some schools and businesses.

 Additional domain controllers Server

If you already have one domain controller in a domain, you can add additional domain controllers to the domain to improve the availability and reliability of network services. Adding additional domain controllers can help provide fault tolerance, balance the load of existing domain controllers, and provide additional infrastructure support to sites. More than one domain controller in a domain makes it possible for the domain to continue to function if a domain controller fails or must be disconnected. Multiple domain controllers can also improve performance by making it easier for clients to connect to a domain controller when logging on to the network. You can add additional domain controllers over the network or from backup media. Before adding domain controllers you should thoroughly understand Active Directory and the requirements necessary to set up additional domain controllers in an existing domain.

Domain Name System

The Domain Name System (DNS) is a hierarchical naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participants. Most importantly, it translates domain names meaningful to humans into the numerical (binary) identifiers associated with networking equipment for the purpose of locating and addressing these devices worldwide. An often used analogy to explain the Domain Name System is that it serves as the "phone book" for the Internet by translating human-friendly computer hostnames into IP addresses. For example, www.example.com translates to 208.77.188.166.

Mail server

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent(MTA) or mail relay is a computer process or software agent that transfers electronic mail messages from one computer to another, in single hop application-level transactions. An MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol. The term mail server is also loosely used to mean a computer acting as an MTA by running the appropriate software. The term mail exchanger (MX), in the context of the Domain Name System formally refers to an IP address assigned to a device hosting a mail server, and by extension also indicates the server itself.

Dynamic Host Configuration Protocol Server

The Dynamic Host Configuration Protocol (DHCP) is a computer networking protocol used by devices (DHCP clients) which dynamically distributes the IP address to the destination host. Dynamic Host Configuration Protocol automates network-parameter assignment to network devices from one or more fault-tolerant DHCP servers. Even in small networks, DHCP is useful because it can make it easy to add new machines to the network.

 Virtual private network Server

A virtual private network (VPN) is a computer network that is implemented in an additional software layer (overlay) on top of an existing larger network for the purpose of creating a private scope of computer communications or providing a secure extension of a private network into an insecure network such as the Internet.
The links between nodes of a virtual private network are formed over logical connections or virtual circuits between hosts of the larger network. The Link Layer protocols of the virtual network are said to be tunneled through the underlying transport network.
One common application is to secure communications through the public Internet, but a VPN does not need to have explicit security features such as authentication or traffic encryption. For example, VPNs can also be used to separate the traffic of different user communities over an underlying network with strong security features, or to provide access to a network via customized or private routing mechanisms.

Distributed File System

Distributed File System (DFS) is a set of client and server services that allow an organization utilizing Microsoft Windows servers to organize many distributed SMB file shares into a distributed file system. DFS provides location transparency and redundancy to improve data availability in the face of failure or heavy load by allowing shares in multiple different locations to be logically grouped under one folder, or DFS root.