A site is a grouping of machines based on a subnet of TCP/IP addresses. An administrator determines what a site is. Sites may contain multiple subnets. There can be several domains in a site.

Active Directory replication to various sites is performed using Active Directory Sites and Services. (Make section explaining how to use this). Sites and subnets are not related to the structure of the domain.

The following may be created:
· Sites – One or more IP subnets. Generally this refers to a physical site such as a portion of the organization in particular city or part of a city which is linked by leased lines or other media to other parts of the organization.
· Subnets – Subnets must be created in each site object before it is really active. A network address and subnet mask is used to define the subnet.

· Site links – It is a list of two or more connected sites. Whether the link will use RPC or SMTP for passing data must be determined before creating the link since it cannot be changed. Selection IP means selection RPC over IP. Site link information includes:

o Replication schedule – Specify the times the sites can replicate and how often they attempt replication.
o Link cost – High for a low bandwidth link. A high cost link gets lower priority. A lower priority link is normally used if there are more than one link to the same location.

o Member sites – Lists sites that are connected using the site link.

o Transport Mechanism – RPC or SMTP (Mail) is specified.

§ SMTP (Mail) – It cannon be used for replication inside the same site and is a form of asynchronous replication.

§ RPC – Requires more bandwidth than SMTP.

Bridgehead server – A domain controller that is used to send replication information to one or more other sites across a site link.

· Site link bridges – Allows one site in a string of sites to replicate through one or two sites to a second or third site. These are only used for fine control of how replication will occur across WAN links. This is actually done automatically by AD, without fine control. To use this feature, automatic bridging of site links must be turned off. You must have three sites to create a site link bridge since it takes three sites and two site links to make a string of sites.

· Global catalog servers – The global catalog is a searchable master index with data about all objects in a forest. The global catalog server maintains this catalog. It:

§ Helps Active Directory resources be located by users.

§ During logon, it provides group membership information.

There is one in each domain by default, and the first domain controller in the domain is originally the global catalog server. It is worthwhile to have a global catalog server on each side of a WAN connection if the domain is spread out across a WAN.

If several domain controllers are placed on the network, and later the network is broken into sites, appropriate servers must be manually moved to the appropriate site that they are on. If the domain controller is created after the site is created, the server is placed automatically in the correct site (based on IP address).

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