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Defining IP Addressing and Subnets

Updated: March 28, 2003

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Determine how many additional IP addresses your wireless clients will require, and whether or not to define additional subnets.

To determine the number of additional IP addresses that you will need for wireless access:

  1. Calculate the number of additional IP addresses that wireless users will require:

    1. Determine the average number of wireless clients currently using your corporate network at any given time.

    2. Add to this the estimated number of additional concurrent wireless clients your network will need to support in the future.

  2. Estimate the number of APs (and associated IP addresses) that you will need for wireless network access. For information about how to determine how many wireless APs to deploy, see "Designing Wireless AP Location" later in this chapter.

Based on the number of IP addresses that you will add to accommodate your WLAN, decide whether or not to add additional subnets.

Creating separate subnets for your wireless networking components offers many benefits, including:

  • Wired network components do not have to draw from the same pool of existing IP addresses as your wireless clients.

  • IP addresses for wireless clients are easier to identify, which assists in easier management and troubleshooting.

  • Separate subnets give you increased control over DHCP lease times.

  • You can associate each of your physical subnets (both wireless and wired) with sites within Active Directory, which enables you to assign network access policies to the specific subnets.

  • If all APs are on the same subnet, you can provide seamless network-layer roaming for the wireless clients. Network-layer roaming allows a wireless client to associate with a new AP within the same subnet, in the same wireless network. When crossing subnets, applications that cannot handle a change of address, such as some e-mail applications, might fail.


  • Network-layer roaming is to be distinguished from general roaming, which allows a wireless client to associate with a new AP within the same wireless network. In network-layer roaming, the wireless client associates with a new AP on the same subnet, within the same wireless network.

Example: An Enterprise Corporation Designs Subnets and IP Addressing for a WLAN

IEEE 802.11 APs are designed with Ethernet ports and use TCP/IP as a networking protocol. Thus, an enterprise corporation designed their network so that the wireless APs in the building are all attached to the same separate subnet, which is connected to a router.

To avoid using IP addresses from existing subnets, they assigned all of the wireless components to a separate subnet. Because they used a separate subnet for wireless components, the wireless components did not adversely affect the available number of host addresses allocated on previously configured wired subnets.

To keep track of the allocation of IP addresses, they created the IP address numbering scheme shown in Table 11.1. The corporation adopted this numbering convention for all of their buildings that have wireless network connectivity.

Table 11.1   Example IP Address Allocation for IP Subnet


IP Address Device


Servers (terminal, proxy, IAS, and so forth)–172.16.50.x

Wireless APs


Wireless clients

Under this addressing scheme, addresses were assigned in the following manner:

  • Within the IP subnet, they assigned the router connecting to the rest of the network the first IP address of

  • They assigned other devices — such as terminal servers, proxy servers, and IAS servers — addresses from through

  • They assigned the wireless APs sequential IP addresses starting with 11. To make it easier to keep track of the wireless APs, they assigned IP addresses that were 10 digits higher than the wireless AP number. For example, Wireless AP 1 was assigned, Wireless AP 2 was assigned, and so forth.

As is usual practice, they assigned static IP addresses to the APs and any servers on the subnet. To prevent the DHCP server from allocating a static IP address to a wireless client, they created a DHCP scope for the wireless subnet that did not include the assigned servers and APs (the scope range was 172.16.50.x+1 through

They located all APs in a building on the same subnet, which allowed network-layer roaming throughout the building. This also made IP addressing by DHCP servers more manageable. The DHCP server assigned wireless clients dynamic IP addresses.

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