Planning Considerations for Microsoft TCP/IP

If your network does not already use TCP/IP, then you need to develop a comprehensive IP addressing plan for your network. When planning your IP infrastructure, include IP network IDs and subnet masks. Use the information in the following sections to create a workable plan.

IP Address Classes

Choosing which address class to use depends on whether your network is private or connected to the Internet. Network addressing is also determined by the size of your infrastructure, which directly relates to which address range to use. Consider the following when planning IP addresses for your network:

Physical Subnet and Host Inventory    Count the subnets and hosts that you have in your current network and then determine how many you need for your new one by subnetting your IP address space. As you do this, plan ahead for at least five years of growth so that you do not run out of addresses or subnets prematurely. If your network is connected directly to the Internet, you will need an IP address range assigned to you from your Internet service provider. For more information about subnetting IP addresses spaces, see "Internet Protocol Security" in the Windows   2000 Server Resource Kit TCP/IP Core Networking Guide .

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It is important to have only a few TCP/IP systems within your network that are directly connected to the Internet, such as the DMZ. The fewer systems that are accessible from the Internet, the safer your network is from attack.

Private Networks with or without Proxy Connection to the Internet    For private TCP/IP networks that are not connected to the Internet, or are connected to the Internet through a proxy server, you can use any range of valid IP addresses from the Class A, B, or C address classes It is recommended, however, that you use private addresses to prevent a renumbering of your internetwork when you eventually connect to the Internet. The private IP address space is defined as three sets of IP addresses set aside by the Internet Assigned Numbers Authority (IANA). The reserved IP ranges are:

  • through

  • through

  • through

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For more information about private addressing, see RFC 1918. The private network address range shown here uses network prefix notation, also known as Classless Interdomain Routing (CIDR) notation to define subnet masks.

Subnet Masks and Custom Subnetting

With public IP addresses in short supply, you can use customized subnet masks to implement IP subnetting. Custom subnetting is defined either as subnetting, Classless Interdomain Routing (CIDR), or variable length subnet mask (VLSM). With custom IP subnetting, you can go beyond the limitations of default subnet masks and use your IP address range more efficiently.

By customizing the subnet mask length, you can reduce the number of bits that are used for the actual host ID. In some cases, you can use default subnet masks for standard-size class A, B, and C networks. Default subnet masks are dotted decimal values that separate the network ID from the host ID of an IP address. For example, if you have a network segment and are using the class A IP address range starting at, the default subnet mask that you would use is Typically, default values for subnet masks are acceptable for networks with no special requirements where each IP network segment corresponds to a single physical network.

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To prevent addressing and routing problems, make sure all TCP/IP computers on any network segment use the same subnet mask.

You can also show subnet masks with your IP addresses by using network prefix notation. This option allows you to show a shortened version of the subnet mask while still maintaining its value. Table 7.2 describes this process. The underlined bits in Table 7.2 make up the network prefix.

Table 7.2 Network Prefix Length Subnet Masking

Address Class

Subnet Mask in Binary

Network Prefix with Decimal Equivalent

Class A

11111111 00000000 00000000 00000000

/8 =

Class B

11111111 11111111 00000000 00000000

/16 =

Class C

11111111 11111111 11111111 00000000

/24 =