In computer networking, a Media Access Control address, better known as MAC address, is a unique identifier assigned to a network adapter or network interface card (NIC) by the manufacturer for identification. The MAC address can also be called the Ethernet Hardware Address (EHA), hardware address, adapter address or physical address. For email it’s interchangeable, haven’t been successful in changing the AppleID though. I’m allowed to change the primary email address on the account but not the account name itself. I think I’m supposed to sign out of all devices first, maybe that’s the blocker. MAC addresses are very useful in diagnosing network issues, such as problems with IP addresses. MAC addresses are useful for network diagnosis because they never change, as opposed to a dynamic IP address, which can change from time to time.

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Every piece of hardware on your local network has a MAC address in addition to the IP address assigned to it by the local router or server. What exactly is that MAC address for?

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader Vishnu Vivek is curious about MAC addresses and their function:

I understand that IP addresses are hierarchical, so that routers throughout the internet know which direction to forward a packet. With MAC addresses, there is no hierarchy, and thus packet forwarding would not be possible. So, MAC addresses are not used for packet transfer.

I don’t think it sits there for no reason. So my question is, where exactly does a MAC address come into play during a packet transfer?

Where indeed? What is the specific function of the MAC address?

The Answer

SuperUser contributor Werner Henze offers some insight into the function of the MAC address:

What are MAC addresses used for?

MAC addresses are the low level basics that make your ethernet based network work.

Network cards each have a unique MAC address. Packets that are sent on the ethernet are always coming from a MAC address and sent to a MAC address. If a network adapter is receiving a packet, it is comparing the packet’s destination MAC address to the adapter’s own MAC address. If the addresses match, the packet is processed, otherwise it is discarded.

There are special MAC addresses, one for example is ff:ff:ff:ff:ff:ff, which is the broadcast address and addresses every network adapter in the network.

How do IP addresses and MAC addresses work together?

IP is a protocol that is used on a layer above ethernet. Another protocol for example would be IPX.

When your computer wants to send a packet to some IP address x.x.x.x, then the first check is if the destination address is in the same IP network as the computer itself. If x.x.x.x is in the same network, then the destination IP can be reached directly, otherwise the packet needs to be sent to the configured router.

Up to now things seem to have gotten worse, because now we have two IP addresses: one is the original IP packet’s target address, the other is the IP of the device to which we should send the packet (the next hop, either the final destination or the router).

Since ethernet uses MAC addresses, the sender needs to get the MAC address of the next hop. There is a special protocol ARP (address resolution protocol) that is used for that. Once the sender has retrieved the MAC address of the next hop, he writes that target MAC address into the packet and sends the packet.

How does ARP work?

ARP itself is a protocol above ethernet, like IP or IPX. When a device wants to know the MAC address for a given IP address, it sends a packet to the broadcast MAC address asking “Who has IP address y.y.y.y?” All devices receive that packet, but only the one with the IP address y.y.y.y will respond with a packet “It’s me.” The asking device receives the answer and now knows that the source MAC address is the right MAC address to use. Of course the result will be cached, so the device does not need to resolve the MAC address every time.

Routing

I almost forgot to mention: there is no routing based on MAC addresses. Low level ethernet and MAC addresses can only reach every device on the same network (cabled or wireless). If you have two networks with a router in between you cannot have a device in network A send a packet to the MAC address of a device in network B. No device in network A has the MAC address of the device in network B, so a packet to this MAC address will be discarded by all devices in the network A (also by the router).

Routing is done on IP level. Simply seen the router is just doing what I described above in the section “How do IP addresses and MAC addresses work together?”. The router will receive packets for its own MAC address but for a different IP address. He will then check if he can directly reach the target IP address. If so, he sends the packet to the target. Otherwise the router itself also has an upstream router configured and will send the packet to that router.

Of course you can have multiple routers configured. Your home router will only have one upstream router configured, but in the internet backbone the big routers have big routing tables so they know the best ways for all packets.

Other use cases for MAC addresses

1. Network switches store a list of MAC addresses seen at every port and only forward packets to the ports that need to see the packet.
2. Wireless access points often use MAC addresses for access control. They only allow access for known devices (MAC address is unique and identifies devices) with the correct passphrase.
3. DHCP servers use the MAC address to identify devices and give some devices fixed IP addresses.

Have something to add to the explanation? Sound off in the the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

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I just tried to send you the screen shot but it doesn't show up in readable form. I also tried sending it in reply to your email to me but it came back undelivered. Please tell me an email address that I can send the screenshot to. Thanks, Jim

Here is a screen shot showing the problem in Excel from Office 2016 that I just installed the

other day...

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Mohan: Since I can't get the screen shot to display when I drag it here, let me tell you what it shows:

I copied several email addresses into a new worksheet from an older one. They all show ok and mouse over shows mailto: for them, and when I click one it opens my email handler on the MAC as usual.

Then I entered my email address in another cell. As I entered it a ___ small line shows in the cell below the entry. Otherwise, the entry looks ok but when I click on the email address I entered it brings up a message box which says ' ALERT The address of this site is not valid. Check the address and try again'

Hope you can help. Thanks, Jim

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Mohan: Do you have anything further for me on this. Could it be related to the last Excel update?

Let me know. Thanks, Jim

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Jim, could the email address where you're seeing the 'file' behavior be malformed? You may have typed something like 'user.example.com' rather than '*** Email address is removed for privacy ***' or the address may contain a space or odd character.

If you retype this address carefully, does it correctly change to 'mailto'?

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No William. I have tried it with several different email addresses and I am sure my entries are in the correct email format.

Delete Old Email Addresses Mac

I even tried editing an existing email address in my worksheet that was performing correctly. I carefully made a change in the entry by deleting from right to left and making a slight change in the email address in the cell. And, BAM, as soon as I hit enter, my now slightly changed former email address in that cell changed to the problem 'file' format.

I have tried re-booting my iMAC but it still persists.

I can work around it by preceding my entry with mailto: in the cell. Then it performs correctly. But that is a pain.

Perhaps on one else is reporting this problem, and somehow it is happening just to me. If so it is even more puzzling.

Thanks for any help. Jim

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I think I'm reproducing your problem but just a little differently. When I enter an email address the protocol changes to 'http://' instead of 'file://'.

That's a bug. Click the Smile button in the upper right corner of your screen to send Microsoft feedback. It will ask to include a screen shot of the problem but I'm not sure you can capture the window that displays the actual wrong link.

I'll report this too.

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Thanks: The smile button allowed me to send a screen shot. It actually shows a shot of your excel sheet for inclusion with the message. Cool! I was not aware of this.

Seeing that you also got the problem, or very similar one, makes me think it is not just me.

I also found you on Twitter and clicked as a new follower. I don't use twitter much. I tried to twit you a picture of my screen shot, but it went to my twitter page instead of yours. I am on twitter @jimsjod so if you go there perhaps you will see it in photos.

Thanks for your help. Jim

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