Investigation to test the answer How to find the factory default MAC address?
Using Windows 11 Pro 24H2 (OS Build 26100.3037)
Only Ethernet interface with a cable connected is a ConnectX-4. No settings had been changed for the ConnectX-4 since it was installed.
Output from ipconfig /all:
Ethernet adapter Ethernet 5:
Connection-specific DNS Suffix . : lan
Description . . . . . . . . . . . : Mellanox ConnectX-4 Adapter
Physical Address. . . . . . . . . : 98-03-9B-6A-7E-A1
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::9e6f:f55a:3a8a:3e29%5(Preferred)
IPv4 Address. . . . . . . . . . . : 192.168.0.59(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : 08 February 2025 13:42:20
Lease Expires . . . . . . . . . . : 09 February 2025 13:42:19
Default Gateway . . . . . . . . . : 192.168.0.1
DHCP Server . . . . . . . . . . . : 192.168.0.1
DHCPv6 IAID . . . . . . . . . . . : 530056091
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-2E-DC-B4-A2-C8-D9-D2-1F-5F-F6
DNS Servers . . . . . . . . . . . : 192.168.0.1
NetBIOS over Tcpip. . . . . . . . : Enabled
The 98-03-9B OUI of the MAC address is registered to Mellanox Technologies, Inc.
Used PuTTY to ssh into a Linux host, and the Linux host showed a MAC address which matched that from the Windows ipconfig:
$ ip neigh
192.168.0.59 dev ens1f0 lladdr 98:03:9b:6a:7e:a1 REACHABLE
On the Advanced adapter properties the Network Address is blank:
Used Generate Random MAC Addresses to create a MAC address which was entered into the adapter properties, without any colons:
Rebooted Windows. ipconfig /all shows the MAC address in use is that set above, and the DHCP server has allocated a new IP address (192.168.0.60) following the change in MAC address (previous IP address was 192.168.0.59):
Ethernet adapter Ethernet 5:
Connection-specific DNS Suffix . : lan
Description . . . . . . . . . . . : Mellanox ConnectX-4 Adapter
Physical Address. . . . . . . . . : 9C-F8-6C-D4-89-1A
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::70a9:7e54:cf4a:961e%5(Preferred)
IPv4 Address. . . . . . . . . . . : 192.168.0.60(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : 08 February 2025 14:08:20
Lease Expires . . . . . . . . . . : 09 February 2025 14:08:20
Default Gateway . . . . . . . . . : 192.168.0.1
DHCP Server . . . . . . . . . . . : 192.168.0.1
DHCPv6 IAID . . . . . . . . . . . : 530056091
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-2E-DC-B4-A2-C8-D9-D2-1F-5F-F6
DNS Servers . . . . . . . . . . . : 192.168.0.1
NetBIOS over Tcpip. . . . . . . . : Enabled
ssh into the Linux machine, which shows the new IP address / MAC address is REACHABLE and the previous ones are STALE:
$ ip neigh
192.168.0.60 dev ens1f0 lladdr 9c:f8:6c:d4:89:1a REACHABLE
192.168.0.59 dev ens1f0 lladdr 98:03:9b:6a:7e:a1 STALE
Flushing the ARP cache, and only the new IP address / MAC address is shown which is a further indication the MAC address was changed:
$ sudo ip neigh flush all
$ ip neigh
192.168.0.60 dev ens1f0 lladdr 9c:f8:6c:d4:89:1a REACHABLE
Searching registry has Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Class\{4d36e972-e325-11ce-bfc1-08002be10318}\0014 with:
| Name | Type | Data |
|---|---|---|
| NetworkAddress | REG_SZ | 9CF86CD4891A |
| DriverDesc | REG_SZ | Mellanox ConnectX-4 Adapter |
On the adapter properties changed the Network Address to Not Present, which when selected cleared the Value:
Rebooted the PC. ipconfig /all shows the MAC address, and the IP address allocated by DHCP, are back to the original values:
Ethernet adapter Ethernet 5:
Connection-specific DNS Suffix . : lan
Description . . . . . . . . . . . : Mellanox ConnectX-4 Adapter
Physical Address. . . . . . . . . : 98-03-9B-6A-7E-A1
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::9e6f:f55a:3a8a:3e29%5(Preferred)
IPv4 Address. . . . . . . . . . . : 192.168.0.59(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : 08 February 2025 14:26:56
Lease Expires . . . . . . . . . . : 09 February 2025 14:26:56
Default Gateway . . . . . . . . . : 192.168.0.1
DHCP Server . . . . . . . . . . . : 192.168.0.1
DHCPv6 IAID . . . . . . . . . . . : 530056091
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-2E-DC-B4-A2-C8-D9-D2-1F-5F-F6
DNS Servers . . . . . . . . . . . : 192.168.0.1
NetBIOS over Tcpip. . . . . . . . : Enabled
ssh into the Linux machine which also shows the MAC address, and the IP address allocated by DHCP, are back to the original values:
$ ip neigh
192.168.0.60 dev ens1f0 lladdr 9c:f8:6c:d4:89:1a STALE
192.168.0.59 dev ens1f0 lladdr 98:03:9b:6a:7e:a1 REACHABLE
$ sudo ip neigh flush all
$ ip neigh
192.168.0.59 dev ens1f0 lladdr 98:03:9b:6a:7e:a1 REACHABLE
Looking in regedit shows that the Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Class\{4d36e972-e325-11ce-bfc1-08002be10318}\0014\NetworkAddress registry key in which entered the random MAC address is no longer present.