IP Configuration Methodology

Windows automates the process for providing a host IP address by using

1. DHCP manual assignment,
2. DHCP dynamic assignment, or
3. APIPA (Automatic Private IP Addressing) automatic assignment.

Depending upon the type of network device or client in your network, you can use any of these IP configuration methods.

• Manual Allocation
  Some hosts on a network, such as special function servers (WINS and DNS), routers, and NAT devices, require manual configuration of the address, mask, and neighbor or gateway addresses. Server applications that assign IP addresses and resolve names to IP addresses require a fixed IP address, subnet mask, and default gateway. Addresses are usually configured manually for DHCP servers, DNS servers, WINS servers, routers, and non-Microsoft hosts that do not support DHCP.

A system administrator configures the address for a host in the DHCP database. The address is then issued to the host by using the DHCP. Use the manual configuration method for devices that function primarily as servers in a client/server relationship and require fixed IP addresses. You can also configure other network devices, such as routers, with reserved client addresses as well. Reversed client addresses work well until you change out the network card of the network device that has the client reservation. Client reservations are based on the Media Access Control (MAC) address which is "burned" into the network card by the NIC manufacturer. When you change network cards on the network device, the MAC address will change for that device and thus invalidate the Client Reservation on the DHCP Server.
The following series of images describes how APIPA works for a client who does not already have an active DHCP lease:

If you need to refresh your understanding of APIPA, read through the following material. Windows supports a mechanism for automatic IP address assignment for simple LAN-based network configurations. This addressing mechanism, called Automatic Private IP Addressing (APIPA), is an extension of dynamic IP address assignment for LAN adapters, enabling the configuration of IP addresses without using static IP address assignment or installing the DHCP Server Service. (Automatic Private IP Addressing) The Windows function that provides DHCP autoconfiguration addressing. APIPA assigns a class B IP address from 169.254.0.0 to 169.254.255.255 to the client when a DHCP server is either permanently or temporarily unavailable. Designed for small non-routable networks, if a DHCP server becomes available later, the APIPA address is replaced with one from the DHCP server. For example, when a Windows Vista machine starts up, it waits only six seconds to find a DHCP server before assigning an IP from the APIPA range. It then continues to look for a DHCP server. Previous versions of Windows looked for a DHCP server for up to three minutes.

• Configuring the LAN
  5) For the APIPA feature to function properly on a computer running Windows 2000, you must configure a network LAN adapter for TCP/IP and click Obtain an IP address automatically in the Internet Protocol (TCP/IP) Properties dialog box.

Microsoft has reserved IP addresses 169.254.0.1 through 169.254.255.254, and uses this range to support APIPA. Remember that these are classified as private IP address ranges, and therefore are not routable through the Internet.

By default, the APIPA feature is enabled. However, you can disable this feature by adding the IPAutoconfigurationEnabled entry to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services \Tcpip\Parameters\Interfaces\GUID_of_the_adapter subkey in the registry. The IPAutoconfigurationEnabled entry takes a REG_DWORD data type. To disable APIPA, specify a value of 0 for the entry.

• Automatic IP Address Assignment for BOOTP Clients
  Rather than completely eliminating BOOTP or providing and maintaining static address allocations for all BOOTP clients, you can have some DHCP servers automatically allocate IP addresses to BOOTP clients. This form of BOOTP service is sometimes called dynamic BOOTP.
  Dynamic BOOTP is attractive because it enables you to serve BOOTP clients from the same pool of addresses as dynamic clients. It doesn’t require you to create DHCP reservations, yet it isn’t adversely affected if you do. This configuration has the added advantage of enabling BOOTP clients to function correctly even if they roam across multiple subnets or if the network is renumbered. The disadvantage of dynamic BOOTP is that the DHCP server must permanently reserve any IP addresses it assigns to BOOTP clients because BOOTP does not include any mechanism for automatically recovering IP addresses that are no longer in use.

DHCP leases addresses from a scope defined for each subnet. Use dynamic address allocation if the majority of clients on a network support this method and do not require fixed IP addresses. By setting the lease time to infinite, you can also use DHCP to configure clients with a permanent IP address. Setting the lease time to infinite provides flexibility in allocating permanent addresses where the host is DHCP-enabled. However, be aware that the IP addresses assigned in this fashion are not the only permanent IP configuration parameters that will fail to refresh. One of the great advantages of using a DHCP server is that you can also change important network parameters such as the WINS, DNS, and default gateway addresses for large groups of computers without having to visit each computer or manually deleting their leases. If you set DHCP clients with permanent lease configurations,they may not receive updated TCP/IP networking information without additional administrative cost.
Automatic Private IP Addressing APIPA allows a computer running Windows 2000 in a small, single-segment network to select an IP address automatically when DHCP is not available. Click the learning bridge link if you would like to review APIPA.
The next lesson wraps-up this module.

Click the Exercise link below to apply what you know about subnet configuration and design.
IP Configuration Methodology - Exercise