This document discusses Windows dialin client issues with subnet masks,
gateways and domain names.
Ensure that the following have been verified before implementing this
The router should already be able to accept dialin calls from the
Windows client. If you need to configure dialin, refer to the document
Configuring an Access
Server with PRIs for Incoming Async and ISDN Calls.
This document is not restricted to specific software and hardware
For more information on document conventions, refer to the
Cisco Technical Tips
Windows PCs do not get IP information for their dialup (PPP) adapters
using Dynamic Host Configuration Protocol (DHCP). They rely on IP Control
Protocol (IPCP) for that purpose. IPCP is the Network Control Protocol (NCP)
negotiated for IP at the end of the PPP negotiation. IPCP includes options for
negotiating IP addresses and TCP header compression (RFC 1332
Microsoft proposed a set of IPCP extensions (RFC 1877
) to match
their implementation of PPP. These extensions define four more IPCP options
that can be negotiated:
Primary Domain Name Server (DNS) Address
Primary NetBIOS Name Server (NBNS) /Windows Internet Naming Service
(WINS) Server Address
Secondary DNS Server Address
Secondary NBNS/WINS Server Address
Cisco supports all six options that define all the IP information
currently delivered to a Windows PC when using a dialup (PPP) adapter. Refer to
the document Configuring WINS, DNS and
DHCP on Access Servers for more information on specifying the DNS and
WINS server information to the client.
Note: Additional information such as subnet masks, default gateway, and
domain name cannot be passed to the client during IPCP negotiation. This is in
compliance with RFC
1877: PPP IPCP Extensions for Name Server Addresses
This document discusses the effects on dialin connections and possible
The NAS and the Windows PC establish a point-to-point connection that
runs PPP. The PC acts as a host that does not route IP traffic between multiple
interfaces. The PC automatically uses the IP address of the network access
server (NAS) (learned during IPCP negotiation) as the default gateway. The PC
knows that if the destination address does not match the local address, the
packet should be forwarded to the default gateway (NAS) which is always reached
through the PPP link.
Microsoft opted for displaying the address (using winipcfg or ipconfig)
assigned to the PC as the default gateway address. This is not an issue if IP
connectivity through the dial-up adapter is operating correctly.
Note: If the PC client is connected to a LAN and then connects to a NAS
(using dial-up networking), then the PC uses the default gateway of the second
connection. This can result in lost connectivity to the LAN. Refer to the
following Microsoft article for more information:
Troubleshooting TCP/IP LAN and RAS Routing Issues
The subnet mask is not needed in the point-to-point environment of
Microsoft opted for showing the classful mask for that address as the
subnet mask instead of leaving those fields blank. Typically, Windows NT 3.5
displays a subnet mask of 0.0.0.0; NT 3.51 (and higher), as well as Windows 95
and 98, display a classful mask depending on the IP address class, while Win2k
and XP display a mask of 255.255.255.255.
Do not worry about this information if IP connectivity through the
dial-up adapter is operating correctly.
For more information on subnet masks refer to the document
IP Addressing and Subnetting for New
The subnet mask and gateway information is obtained when running the
Windows IP Configuration program (winipcfg) on Windows 95 and 98 machines, or
running the Windows NT Configuration program (ipconfig) on Windows NT, 2000 and
XP machines. The following screen captures are shown as samples:
Since the domain name information cannot be passed during IPCP, there
are three options:
The user must use the fully qualified domain name (FQDN) of the
Manually specify the domain name information in the Windows PC TCP/IP
properties. This may be the only feasible option for a NAS with a large Windows
95 or 98 client base. Use bootp and DHCP to obtain this information after IPCP
negotiation is complete.
The Windows client sends a DHCP inform packet to the NAS, which then
sends back the domain name information. The DHCP functionality can either be on
the NAS itself or an external DHCP server. Currently only Windows 2000 and XP
clients support sending DHCP inform. Use the Microsoft website to verify this.
Configure the domain name within the TCP/IP properties of the client.
Refer to the following Microsoft article for more information:
Q200211-DUN Clients Do Not Receive DNS Domain Name over
Some Microsoft operating systems (for example, Windows 95 and 98) may
not support obtaining domain names from the NAS through DHCP inform. Hence,
manually specifying the domain name on the client may be the only viable
option. However, we recommend that you refer to the Microsoft website to check
whether that functionality is included in the Windows OS version you use.
The router can send additional information to the dialup client using
) after IPCP negotiation is complete.
The Windows 2000 or XP client sends a DHCP inform (option 15) packet to
the NAS. The NAS then responds with the domain name information. The DHCP/bootp
functionality can either be on the NAS itself or on an external DHCP server.
Windows 2000 and XP clients can send the DHCP inform packet after some
changes to the registry. Refer to the following Microsoft article for more
information on the client configuration:
Q312468-How to Request Additional DHCP Options from a DHCP
We strongly recommend that you verify the client configuration
procedure on the Microsoft website prior to making any changes on the Client
Warning: Modifying the Windows registry should only be attempted by
experienced system administrators because mistakes can render the system
unbootable. Refer to the Microsoft website for appropriate precautions.
To configure DHCP on the NAS refer to the following documents:
You can specify the domain name that is to be provided to the client
using the command domain-name within the dhcp pool
configuration. The IOS DHCP feature was introduced in Cisco IOS® Software
You can use an external DHCP server instead to supply the neccessary
domain-name information to the client using bootp. Perform the following steps:
Configure the DHCP server with the domain name attribute. Refer to
the DHCP server documentation for more information on specifying this option.
Configure the command ip helper-address
on the the Group-Async interface
(for modems), or the Serial x:23 (d-channel)or Dialer interface (whichever
controls the call) for ISDN calls as appropriate. The address should specify
the IP address of the DHCP server that the bootp request is to be forwarded to.