Hello, and welcome to this lecture on MAC addresses. In the last lecture we looked at ethernet and we also looked at CSMA/CD. In this lecture we’re gonna talk about a layer two concept known as MAC addresses.
First up, what is a MAC address? MAC address is a 48-bit address that is burned onto the network devices. It is known as the real address of a device because it is programmed onto the network chip. It is also known as the physical address of a device.
When you think about IP addresses for a second, IP addresses are considered to be logical addresses because you can assign whatever IP addresses you like to your devices. However, the MAC address is programmed into the chip or into the network device, hence it is called as the real or the physical address of the device. It stands for media access control address. Let’s talk about the MAC address format. I have an example for you on the screen. The example MAC address is 01:AC:2F:D3:B4:23.
Presentation of Address
The MAC address is represented as six groups of two hexadecimal digits separated by colons. You may already know that hexadecimal characters can range from zero to nine and A to F, and every hexadecimal character when converted into binary takes four bits. So we have 12 characters. 12 times four results in a 48 bit address. The first three groups or six characters are together known as the organizational unit identifier or OUI.
It identifies the manufacturer of network equipment. Let’s now talk about broadcast MAC address. A MAC address that consists only Fs, so all the 12 characters are only Fs, is known as a broadcast address. When you convert that into binary, every F would be represented by four ones. So when you convert that into binary you would have a MAC address of only ones or a address of 48 ones.
When a frame is sent with a destination set as the broadcast address, the frame would reach all the host on the same layer two network. We also have something called as multicast addresses. The OUI of a multicast MAC address is always set to 01:00:5E. So we just discussed that every hexadecimal character can be represented by four binary bits, and we have six over here. So six times four is 24.
So 01:00:5E take up the first 24 bits. The 25th bit has to be a zero. As you can see in the diagram, the first 25 bits are indicated in binary format. So you only have the remaining 23 bit to play with. To get the lowest possible multicast address we can put in all the zeros. That would give us an address like 01:00:5E:00:00:00.
To get the highest possible MAC addresses we can put all ones in the remaining 23 bits, and that would give us a MAC address like 01:00:5E:7F:FF:FF. So that’s it for this lecture. Let me know if you have any questions. If not, I’d like to thank you for watching, and I’ll see you in the next lecture.
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