There is only one really important point, which is the overriding factor when designing a replication strategy for your network: how much traffic and over what period will you be replicating across the network? However, replication isn’t the only reason for creating sites. Sites also need to exist to group sets of machines together for ease of locating data, finding the nearest DC to authenticate with, or finding the nearest DFS share mount point.
Before you sit down to design your site and WAN topology, you need to obtain the map of your existing network infrastructure. This map should contain all physical locations where your company has computers, along with every link between those locations. The speed and reliability of each link should be noted.
If you have an existing IP infrastructure, write down all the subnets that correspond to the sites you have noted.
From the network diagram, you need to draw your site structure and name each site, using a one-to-one mapping from the network diagram as your starting point. If you have 50 physical WAN locations, you have 50 sites. If only 30 of these will be used for Active Directory, you may not see a need to include the entire set of sites in Active Directory. If you do include the entire set, however, it is much easier to visualize your entire network and add clients or servers to those locations later.
When drawing Active ...