Microsoft KB Archive/160699

= Understanding DHCP IP Address Assignment for RAS Clients =

Article ID: 160699

Article Last Modified on 2/22/2007

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APPLIES TO


 * Microsoft Windows 2000 Server
 * Microsoft Windows 2000 Advanced Server
 * Microsoft Windows 2000 Professional Edition
 * Microsoft Windows 2000 Datacenter Server
 * Microsoft Windows NT Workstation 3.5
 * Microsoft Windows NT Workstation 3.51
 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT Server 3.5
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0 Standard Edition

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This article was previously published under Q160699





SUMMARY
When Remote Access Service (RAS) uses Dynamic Host Configuration Protocol (DHCP) to obtain IP addresses for dial-in clients, only the IP address from the DHCP lease is passed to the RAS client. Other options in the DHCP scope are not. This article describes the behavior that occurs.



MORE INFORMATION
The RAS help file states, "Use DHCP to assign remote TCP/IP client addresses."

RAS servers can obtain IP addresses for remote clients from a dynamic host configuration protocol (DHCP) server. DHCP servers provide static and dynamic IP address allocation on a large network. You should select this option if a DHCP server is available.

When the RAS server starts up with the option to Use DHCP to assign remote TCP/IP addresses, it makes several DHCP requests in advance and caches the DHCP leases that it will need for dial-in clients. The RAS Server will request a number of addresses according to the behavior defined in the following article:

216805 RAS Server Behavior When Configured to Use DHCP [ntrelease]

The RAS server records the address of the DHCP server, the leased IP address, when the lease was obtained, when the lease expires, and the lease length. All other DHCP options defined for that scope are discarded. When the client dials into the RAS server and requests an IP address ("Server Assigned IP Address" is selected) the RAS server will use one of these cached leases. The IP address is then given to the dial-in client, which is unaware that the IP address has been obtained through DHCP. The RAS server maintains the lease on behalf of the client. Therefore, the only information that the client receives from the DHCP lease is the IP address.

When a RAS client obtains an IP address Lease from a RAS server, Winipcfg.exe (for Windows 95) or Ipconfig.exe (for Windows NT and later) displays the following information about the Lease:   Lease Obtained:      Tue Jan 1 80 12:00:00 AM   Lease Expires:       Tue Jan 1 80 12:00:00 AM When a RAS server assigns an IP address to a RAS client either from a Static Address Pool or DHCP, there is no real lease time for the IP address since it will be released when client disconnects.

However, RAS clients can still receive additional TCP/IP configuration information from the RAS server. WINS server assignments and Domain Name Service (DNS) server assignments can be delegated to the client when it connects.

Again, this information is not taken from the Options given in the DHCP lease. Instead, this information is taken directly from the RAS server's settings. If a RAS server has WINS or DNS entries, these entries are passed to the client.

Here is a list of the DHCP Options that Microsoft DHCP clients can support and how a RAS client obtains them:

IP
As described above, the RAS server obtains an IP address from the DHCP server. The RAS server then gives the IP address to the dial-in client and manages its lease. This is the only information from the DHCP server that the RAS client receives.

WINS
This is taken from the RAS server if the RAS server is configured with WINS addresses. The client acquires the list of WINS servers that are configured on the RAS server.

DNS
This is taken from the RAS server if the RAS server is configured with DNS server addresses. The client acquires the first DNS server address listed in the RAS server's DNS Service Search Order.

Subnet Mask
The subnet mask corresponds to the standard mask associated with the standard class type of the given IP address.

NOTE: Note that Windows 2000 clients use a subnet mask of 255.255.255.255 (not the classful mask for the IP address) on PPP links.

NetBIOS Scope ID
NetBIOS scope ID information is not passed to the client. If you need to modify this setting it will have to be changed directly on the client.

Node Type
Node Type is not taken from the DHCP lease but can change on the RAS client depending on WINS information. If the RAS server has no WINS servers defined locally, a b-node Windows NT RAS client remains a b-node client. If the RAS server has WINS servers defined locally, a b-node Windows NT RAS client switches to h-node for the duration of the connection.

NOTE: Windows 95 clients do not automatically switch between node-types if the RAS server supplies WINS addresses. This must be done manually: Click Control Panel and double-click Network. On the Selection tab, select TCP/IP from the list of installed network components and then click Properties. Click the WINS Configuration tab, and then either manually specify the WINS addresses or click Use DHCP for WINS Resolution.

An IP address obtained from a dial-up connection cannot be released or renewed with the Winipcfg.exe utility in Windows 98 or Windows 95. The Winipcfg.exe utility can only release IP addresses obtained from DHCP.

Because dial-up clients obtain IP addresses from the Remote Access Service rather than from DHCP, dial-up clients retain this address until the session disconnects.

For additional information, click the article numbers below to view the articles in the Microsoft Knowledge Base:

121005 DHCP Options Supported by Clients

124358 RAS Server Assigns Cached IP Addresses to RAS Clients

142303 DNS Address from RAS Server Not Passed to RAS Client

160177 Default Node Type For Microsoft Clients

Keywords: kbinfo kbnetwork KB160699

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