MAC addresses in router vs Access Point

The premise is wrong. Where did you get that impression?

Neither access point nor router 'has' MAC addresses. It's ethernet frames that have MAC addresses, two each, one indicating a source, and one indicating a destination. Several frames coming out of some ethernet port can have different destination MAC addresses, and even different source MAC addresses, e.g. when the port is a port of a bridge device.

best regards Patrick

Where did "you" get that impression?

If a router interface didn't have a MAC address, how do you think you would communicate with it?

When your host sent this post, it forwarded its "framed" packets to the default gateway's MAC address.

As the "framed" packets traverse the Internet on their way to the NNTP server, the source and destination MAC addresses in the frames changed hop-by-hop to reflect the MAC addresses of the routers involved at each hop. Only the source and destination IP addresses remained the same.

Even switches have a different MAC address for each port, as they are the "source" of some protocol traffic. Look at STP packets sent on each port with a sniffer.

Best Regards, News Reader

By noticing that such addresses are stored in EEPROM for software to read out, but the it's totally up to the software which address is in each an every frame.

I send it a frame. When it replies somehow, I communicated. In principle the router software could just accept all incoming frames (called promiscous mode, usually) and operate on them regardless of their source or destination MAC addresses. A packet sniffer does that.

I know how IP works, thank you. I'm not at all talking about IP here.

I know.

best regards Patrick

I'll take your statement literally, and assume you meant router WAN port (as stated), and not WLAN port (since you were comparing to a WLAN device).

The router "would" have a unique MAC address for each interface (LAN, WAN).

What lead you to this belief?

Best Regards, News Reader

No doubt his device has a sticker on it listing MAC addresses with which it is configured. No doubt, he accesses an administrative interface on the router using an assigned MAC address. You want to argue whether it "has" MAC Addresses?

Do you really think your response was helpful to this individual, basing it on some esoteric example, and leaving him with no explanation to gauge your response?

When you don't explain the basis for your statement, how are we to gauge your knowledge?

Best Regards, News Reader

Somewhere back in the origins of ethernet is the ability to assign MAC addresses either to ports or hosts.

The only one I know to commonly apply them to hosts is Sun, which used to put the address in ROM on the CPU board which may or may not have an ethernet interface. That address was then used on all ports on that host. (Sun machines were often configured as routers with more than one ethernet port.)

Since SOHO routers are sometimes used with systems that depend on a specific MAC address, such as some cable systems, they usually have the ability to specify the WAN port address. That address will, then, often be assigned the same value as one of the hosts on the LAN, so it should not be used on the LAN interface.

Do you know of any small routers that only have one MAC address?

-- glen

Nice refresher.

The MAC address "cloning" negates the need to reconfigure on the ISP's side when the router is introduced to the topology, after a host was previously connected directly to the ISP.

i.e.: The transition is transparent to the ISP because they still see the original (now cloned) MAC address.

The cloned host with it's original MAC address can coexist since it and the WAN interface (using the cloned MAC address) are on separate collision domains.

Best Regards, News Reader

Yes, I think it might have been a bit helpful. Especially the first part, where I told them their premise was wrong. The semantic game I then played for a short time might have been helpful or not, I don't know.

Now, what about you debating me instead of answering what was asked by the original poster?

HAND Patrick

That was also my reaction to the original post. It is possible that the router might have only one MAC address assigned to it, if it follows that somewhat iffy (IMO) Sun philosophy you mention.

Note this, however. Access Points are not necessarily combined with a router function. Therefore, the two interfaces of an access point

*could* exist within the same network (IP subnet). Whereas the two interfaces of the router would, by definition, each be in its own IP subnet.

Therefore, it's acceptable to assign just one MAC address to a router, but not to the access point.

Another minor point is that sometimes switches use the MAC address of one of their interfaces as the MAC of the entire box, if a single MAC address to identify the box is needed. I'm too lazy to look it up right now, but doesn't RSTP do this, for example?

Bert

So it would be just a bridge. I have thought about connecting just to the LAN port on a router/access point to put the WLAN on the same subnet as an existing wired LAN.

Does a switch without spanning-tree need any MAC address? If so, does it need more than one?

-- glen

But without a router function, why two addresses?

News Reader wrote: (snip)

Yes, but not if it used the same MAC on both the WAN and LAN ports, as seems to be allowed for IP routers.

-- glen

By "switch," I did mean bridge. Sorry for not being more specific.

I suppose that a managed switch, implementing no spanning tree protocol, would need at least one MAC address, so that it can be reached by an IP host. Other than that, should be similar to a hub (repeater), in terms of addressing needs.

Hmm. Good point. If the access point has no router function, one could consider it to be a mere repeater. My reason for saying that it needs two MAC addresses was only because this is the only way to allow unambiguous addressing of the wired and wireless interfaces, even if in the same IP subnet. I suppose you could argue that addressing the two interfaces is not mandatory at all. So I stand corrected.

Bert

On a WAN port, a router often does not use ethernet framing at all. In these cases no (ethernet) MAC address is used on the WAN port.

best regards Patrick

Excuse me?!?!?

Read up on (for instance) serial links and PPP encapsulation. No MAC address is used.

Steinar Haug, Nethelp consulting, snipped-for-privacy@nethelp.no

But for PPPoE, as you might find in a router with wireless access point, there would be a MAC address. I believe there are enough home routers using ethernet that 'often' doesn't apply to serial links anymore. (I presume you mostly mean the serial links used for T1 connections.)

-- glen

You mean, as you use to interface something found in a home with the DSL modem. The WAN side often ends up being ATM again, and probably doesn't use ethernet framing. ``Probably'' because this is an educated guess and I have not looked at what actually comes out on that side of the device.

In sheer numbers, most ``routers'' might be DSL modems fancied up with some other functionality, but I somehow fail to think of last mile linkups as Wide Area Networking. T1/E1 indeed don't use ethernet framing, and neither does plain dialup, but plenty other technologies also don't (think SONET, etc.) altough you can often enough transport ethernet frames over those again.

The point was that ethernet MACs are not inherently required for ``networking'', and thus network ports will exist without a MAC address. Logically, those ports will not be ethernet ports, but that doesn't mean you can't push packets through them.

an Ethernet device has a MAC to facilitate media communications. No MAC - no communications.... hence the acronym. This MAC may be merely copied from other incoming data frames, as with a hub, switch, or bridge.

However, if the device is "smart" and needs to be addresses via IP or some other higher protocol, the device really needs a MAC of it's own, so any address resolution (ARP) can find it on the network.

If a general home router has a a LAN and WAN interface - both Ethernet - then it will probably have 2 MACs, one for accessing the router from each "world".

Other high end routers (or switches) have a MAC, and a corresponding IP address to access and manage the entire device.

just a note to the OP - what device are you talking about ? ie - on my Linksys WAP 54G WiFi access point (not a router), the same MAC is used for the LAN and the WiFi (WLAN).

but on my Linksys router (no WiFi) it has a different MAC for the LAN (4 port switch) and for the WAN (connected to DSL modem)

Don't confuse WLAN (Wifi) vs WAN (wide area network = cable or DSL)