I have 2 wireless cards with the same MAC address at the hotel

Because the packets are not routed to a mac address they are route to an IP Address.

Are they connected to same access point?

--PA

It's because a NAT device such as a router is being used. The router has a DHCP server that issues an unique IP to each NIC requesting that an DHCP IP be issued to it. NAT is mapping technology it maps traffic back to the requesting IP/machine's NIC that sent outbound traffic to a remote IP on the Internet. Not only does the router apply the MAC in the network traffic when it sends traffic outbound, it also applies the LAN IP that has been assigned to the NIC too in the traffic, so that the inbound traffic can be mapped back to the requesting machine. Inbound traffic is not mapped back to a machine, unless there is corresponding traffic sent outbound by the machine, which has a LAN IP.

Duane :)

If you assigned the MAC on one, and left the other in it's original form, the MAC's are actually one bit apart: The last bit indicates that the MAC was set and not the factory MAC address.

At the network level, maybe, but not at the data link level. You might want to read up on the OSI 7 Layer Model before making such statements.

Does not compute! MAC is a hardware address "permanently burned-in" by the manufacturer. If Windows detects two identical MACS a system event log entry is generated and network connectivity cease.

Following also quoted from Knowledge Database Article ID 164903: "Another source of duplicate media access control addresses can occur if you are assigning locally administered media access control addresses (LAA), in which case you are overriding the burned-in address in favor of the locally assigned media access control address. With some drivers, LAA media access control addresses are possible, usually Token Ring adapter drivers."

Keyword: "seem"

Run Ethereal or some SNMP utility and you'll probably find quite a bit of collisions going on...

Not necessarily. Some devices, such as some high end network cards and SOHO routers, have the ability to set the MAC address. One of the MAC address bits, IIRC, indicates whether the MAC was set by human intervention or the one it shipped with.

Drive-by replying?

Read his post again...

("so I used MacMakeUp to make both computers have the same MAC address")

That computes how he could have two NIC's with the same MAC.

They are within five feet of each other so I presume they connected to the same free wireless access point in the hotel.

Hi Nat, Does this mean that each computer gets TWO unique addresses? The MAC and the IP.

Are you saying that even though the MAC address is the same, that the combination of Mac + IP address is different because the IP addresses are different?

Many wireless networks can occupy the same airspace. You need to doublecheck your connection properties to make sure they really are on the access point you expect to be on.

Oh, Do you mean the same SSID? I had not thought of the fact they 'could' be connected to different SSIDs. But, in this case, the SSID is definately the same for both Windows XP PCs as it's the only free SSID available.

I'm checking out at noon today (on my way to South Africa) so I won't be able to respond for a while.

Jette Goldie

Is the MAC address "really" burned in? MacMakeUp seems to "change" the MAC address at will.

I just checked the WinXP System Event Log by right clicking on "My Computer" and selecting "Manage" and then selecting "System Tools", "Event Viewer", "System".

I am not sure what to look for, but, I did see...... The system detected that network adapter \\DEVICE\\TCPIP_{BAACBE02-D4AF-43D0-BF89-D45C0B039834} was connected to the network, and has initiated normal operation over the network adapter.

And ... Your computer was not able to renew its address from the network (from the DHCP Server) for the Network Card with network address D45C0B039834. The following error occurred: The semaphore timeout period has expired. . Your computer will continue to try and obtain an address on its own from the network address (DHCP) server.

This may or may not be a red herring because the sysytem "seems" to be working well.

Jette

The "MacMakeUp" freeware program will change the MAC address for any Windows PC network card.

Can you tell from their web site whether the "human intervention" switch is also reset?

Jette

No, there is no way to force the clone bit off without using the original MAC address programmed in that NIC's ROM. The clone bit only usually comes into play for identification is on routers and wifi points smart enough to know about the bit, in the case that two devices on the network appear to have the same MAC, and even then it's not foolproof if more than one device is cloned on the same network.

Some cable and DSL providers have been known to not allow devices that have the clone bit set to talk to the cable modem or DSL bridge/router, but that's a fairly rare case.

Yes, there's usually some method to revert a programmable network device back to it's original MAC.

You have a programmable network card and are telling it to spoof a different MAC address.

I don't know enough about Windows error messages to know what it's calling a semaphore timeout (though I wouldn't mind learning). Only thing I can think of is that your machine sent a DHCP request, the DHCP server responded with an answer and the other machine with the same MAC address answered first, the first machine then ignores the acknowledgement it wasn't expecting to get out of order and times out, however, this is just a theory I can't prove without a packet analyzer on the same network you're on. I don't have a lot of confidence in my hypothesis though: Most people make it a point not to put two of the same MAC address on the same network.

More than one access point can have the same SSID, and to enable seamless roaming this is necessary. What you would want is to compare the Associated MAC address or BSSID of the connected access points.

What I am telling you is that NAT router knows from what internal LAN IP data is coming from and knows where to send the data back to the LAN IP.

The MAC comes into play for the DHCP server on the router to assign an IP to a MAC. I don't think it matters if a same MAC is in the DHCP table, as the router is applying other things in the identification as to who is who.

Since the router has built in swicth technology, then the MAC comes into play in broadcasting the data directly to a NIC's MAC. Otherwise, it may be broadcasting traffic to the duplicate MAC(s) acting as a dumb hub sort of speaking where the LAN IP has to be coming into play. There could be traffic collision on inbound and outbound traffic for the two machines due to the simple broadcasting of data to them as a hub, instead of finding the MAC that the data belongs too acting like a smart switch.

How else is the traffic making it back to the machines if the machines have two NIC's with the same MAC(s)?

Duane :)