Windows 95/98

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Windows 9X Networking

Automatic Private IP Addressing

Windows 98 incorporates a new networking feature referred to as Automatic Private IP Addressing. This feature is incorporated into the operating system to provide automatic IP address assignment in the absence of a DHCP server. This is mainly for small LANís that cannot support the overhead required for a DHCP server.

Automatic Private IP Addressing (APIPA) is enabled through the following process:

  1. TCP/IP is set to obtain an IP address automatically
  2. Windows 98 attempts to locate a DHCP server for the address assignment
  3. If Windows 98 fails to locate a DHCP server, or the DHCP lease for the address has expired, then Win98 will assign itself an IP address in the form of 169.254.x.x (Class B). The subnet mask is 255.255.x.x

The familiar DHCP error message from Windows 95 "DHCP server was not found" has been turned off in Windows 98 by default. It can be reenabled in one of two ways:

  • If the Windows 98 machine ever receives a response from a DHCP server, then the error message is turned on. This should ensure that only those machines that expect to receive an IP address from a DHCP server would ever indicate that it could not lease an address.
  • DHCP error messages can be turned on and off via the registry.

Scenarios involving APIPA

  1. Boot: No previous IP / No DHCP Server:

    • - the system will do a Discover three times, if it cannot find the DHCP server, it will assign itself the Class B address

    • - it will not show an error message (assuming it has never seen a DHCP server

    • - it will do a discover every three minutes

  2. Boot: Previous IP / No DHCP Server:

    • - the system will check for the DHCP server, if it cannot find one, it pings the default gateway

    • - if the default gateway replies, then the system will keep the previous leased IP address

    • - if it cannot find the default gateway, then it will assign itself an IP address

    • - it will show an error message

    • - it will do a discover every three minutes

    • - if a DHCP server comes on line later, the user will get a dialog that states communication has been reestablished with the DHCP server

  3. Boot: Lease Expires / No DHCP Server:

    • - system will try to reestablish the lease of the IP address

    • - when no DHCP server is found, it will then assign itself an IP address

    • - it will show an error message

    • - it will do a discover every three minutes

    • - if a DHCP server comes on line later, the user will get a dialog that states communication has been reestablished with the DHCP server

    WinIPCfg

    WinIPCfg has been modified to report whether APIPA or DHCP generated the IP address. If APIPA is utilized, then the "IP Address Obtained By" entry will read Automatic Private.

    Limitations

    While APIPA can automatically self-assign a TCP/IP address to a Windows 98 system, it does not generate all of the information typically provided by a DHCP server, such as DNS and WINS Server addresses. Therefore, a system relying on APIPA can only resolve domain and NetBIOS names via broadcasts. Currently, APIPA systems can only communicate with systems that also have an APIPA address.