Subnetting in Windows Server 2008



Each host, in addition to an IP address, needs a subnet mask. The subnet mask, like an IP address, is a 32-bit value typically expressed as four octets separated by periods. The subnet mask serves to mask the IP address into its two components, network ID and host ID, which allows traffic to be routed to the appropriate network and then to the destination host. Table 1 shows the subnet masks for the three standard network classes.

TABLE 1: Standard Subnet Masks



In addition to masking the host ID from the network ID, a subnet mask also can serve to segment a single network into multiple logical networks. For instance, assume that your small company gets Internet access from a local ISP. The ISP uses a class C address space to accommodate a group of small business clients, of which your company is one. The ISP uses a subnet mask of 255.255.255.192 to divide the network into eight subnets with 30 hosts each. Table 2 lists the host ranges for each subnet.

TABLE 2: Sample Subnet



In this example, the ISP uses the first address range (subnet 0) for a routing cloud (a network subnet that functions solely for the purpose of routing), and the remaining seven subnets to accommodate the customers. You are the first customer and you get subnet 1, with addresses from 33 through 62. Figure 1 illustrates the network.

As you are designing your network and assigning IP addresses and subnet masks, remember that all nodes on the same logical segment need to have the same subnet mask. This places them in the same logical network for routing purposes.

Note: A full understanding of subnetting is essential for deploying Active Directory across multiple sites in an enterprise or the Internet.