What Site am I in?
Each time an AD client boots up it figures out what site it is in: It checks its IP address against the AD subnet objects - the client then knows it is in the Site that its subnet is associated with.
When a server is DCPROMO'ed, DCPROMO figures out, based on the server's IP , what Site the new DC is in. This is same process the client does, but this is only done at DCPROMO on DCs, whereas it's done on every boot-up on clients. If a DC is re-IP'ed or physically moved, you must manually move it's DC object in AD Sites and Services to the new appropriate Site object - assuming it needs to be changed.
Create Sites and Subnet objects even for subnets where there are no DCs, as clients use these to locate themselves.
Site / Network Topology
AD objects that represent the physical (have a physical equivalent):
AD objects that represent the logical (do NOT have a physical equivalent):
AD Sites and Site Links create a site topology and represent the network topology - the Sites and Site Links do not necessarily map one-to-one to the physical network.
Sites and Site Links are used by AD to choose what A to B client-server services take or what A to B replicates, but the actual LAN/WAN network infrastructure actually routes the traffic by following its own protocols, regardless and unaware of the AD site topology.
If all nodes on the network can communicate with all other nodes on the network, the network is called meshed or fully routed.
The Site Object
The AD site maps to or represents or models the physical network on which AD runs. AD object-wise it is the most important representative of the physical network. The logical and physical structures of AD are independent of each other. Sites are NOT automatically discovered or created - this is a manual process.
A Site is made of one or more well connected IP sub-nets, a region of the network infrastructure . Well connected generally means >=10MB. A Site is basically a LAN. Site break AD up to follow LAN vs. WAN topology of the network and are the mechanism by which AD is aware of the infrastructure it is on.
Sites are used to ensure users access certain resources preferentially. Sites allow clients to access resources (DCs, GCs, DFSs, etc.) in their same site (ie LAN) instead of resources across the WAN. In other words, client-server traffic is localized.
By default the first DC in the forest's dcpromo creates a single site named Default-First-Site-Name into which everything goes. This default initial Site can be renamed. This default initial Site becomes the default Site for any DCs added later for which other Sites are not specifically specified.
Sites are used to control AD replication traffic.
In creating a Site, a Site Link must be specified too. (DEFAULTIPSITELINK can be used)
The Subnet Object
The Subnet Object defines subnets (or networks) on your network. Subnet objects are associated with Sites.
The Site Link Object
By default the first DC in the forest's dcpromo creates a single Site Link named DEFAULTIPSITELINK.
Site Links are used to model the amount of available bandwidth between Sites.
Site Links represent one or more physical connections between Sites - not a specific connection, just that there is some connection.
A Site Link must have at least two Sites.
Site links control replication.
Site links control how clients and servers determine the closest site.
Site links have parameters including:
Site Link Bridges
Site Link Bridges contain Site Links, not Sites. With Bridge All Site Links ENABLED, all Site Links are in one (fully meshed) Site Link Bridge.
AD assumes and by default sets the Bridge All Site Links to ENABLED - this assumes a fully meshed or routed network.
With Bridge All Site Links set to ENABLED - Site Link Bridges are not needed.
With Bridge All Site Links set to DISABLED, a non-fully-meshed network is specified and AD assumes that only Sites with Site Links between them can communicate.
With Bridge All Site Links set to DISABLED, Site Link Bridges can be created/added.
RPC over TCP/IP and SMTP each have separate and independent Bridge All Site Links settings.