Custom DHCP Client Identifiers for Windows NT
ID: Q172408
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The information in this article applies to:
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Microsoft Windows NT Workstation version 4.0
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Microsoft Windows NT Server version 4.0
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Microsoft Windows 2000 Professional
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Microsoft Windows 2000 Server
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Microsoft Windows 2000 Advanced Server
SUMMARY
A Dynamic Host Configuration Protocol (DHCP) client will normally use the
media access control address of its network interface card (NIC) to
identify itself to a DHCP server. Windows NT DHCP clients can also have
client identifiers manually configured.
MORE INFORMATION
When DHCP clients request and renew IP addresses, they normally send their
hardware type and media access control address to a DHCP Server as a unique
identifier. This allows the DHCP server to track the allocation of
addresses and allows administrators to reserve addresses for specific
interfaces. This client identifier is sent as option 61 in the client's
DHCP Discover and Request packets as described in RFC2132, "DHCP Options
and BOOTP Vendor Extensions." In general, this identifier will be the media
access control address of the network interface card requesting the
address, but this is not required.
Most implementations of DHCP are simplified by allowing the Windows NT
client to automatically specify their media access control address, but it
is possible in Windows NT 4.0 to manually enter a client identifier. This
allows an administrator to reserve an IP address for a DHCP client so the
client will receive the same IP address even if its network card is
replaced.
Implementing a user-defined DHCP client identifier requires Windows NT 4.0
SP2 or later. To manually configure a DHCP client identifier, perform the
following steps:
WARNING: Using Registry Editor incorrectly can cause serious, system-wide
problems that may require you to reinstall Windows NT to correct them.
Microsoft cannot guarantee that any problems resulting from the use of
Registry Editor can be solved. Use this tool at your own risk.
- Start Registry Editor (Regedt32.exe) and select the following subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\
<Adapter Name>'X'\Parameters\Tcpip.
where <Adapter Name> is the network adapter driver name and 'X' is the
number of the adapter. For example, for an Intel Etherexpress Pro
network adapter the <Adapter Name> is Epro so you might have an entry
similar to Epro1.
- Click Add value from the Edit menu, and type the following information:
Value Name: DhcpClientIdentifier
Value Type: REG_DWORD
Value Range: 0x0 - 0xFFFFFFFF
Value Default: 0x0
- Click OK, exit Registry Editor, and restart the system for the changes
to take effect.
Creating a DHCP Server Reservation
A DHCP server can use a custom DHCP client identifier to reserve an IP
address for a specific network interface card. If the NIC is replaced, the
new NIC will receive the same IP address from the DHCP server. When a
Windows NT DHCP client sends a client identifier to a DHCP server, it sends
the identifier as 4 bytes, or 8 hexadecimal characters. The identifier is
sent in groups of two hexadecimal characters, with these groups of two sent
in reverse order. If the identifier is less than 8 hexadecimal characters,
then zeros are padded at the end of the identifier. For example:
Custom Client Identifier Client Reservation on DHCP Server
12345678 78563412
123456 56341200
1234 34120000
1234567 67452301
12345 45230100
123 23010000
A18F42 428FA100
CF432 32F40C00
C32D1BE BED1320C
Refer to the online Help in DHCP Manager under the section "Managing Client
Reservations" and pages 206-207 of the Windows NT 4.0 Server Networking
Guide.
REFERENCES
For more information on DHCP reservations, see RFC1700 and RFC2131.
RFCs may be obtained through the Internet as follows:
Paper copies of all RFCs are available from the NIC, either individually or
on a subscription basis (for more information contact NIC@NIC.DDN.MIL).
Online copies are available via FTP or Kermit from NIC.DDN.MIL as
rfc/rfc####.txt or rfc/rfc####.PS (#### is the RFC number without leading
zeroes).
Additional query words:
CID
Keywords : kbnetwork nthowto ntnetserv nttcp NTSrvWkst
Version : WINDOWS:2000; winnt:4.0
Platform : WINDOWS winnt
Issue type : kbinfo