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MAC Addressing Formats And Broadcasts


Your CCNA exam success depends on your mastery of networking, routing, and switching fundamentals. Those fundamentals have to include knowledge of MAC addresses, so let's take a close look at Media Access Control addressing.

A typical MAC address looks something like this:

af-14-b3-c2-14-45

You may be wondering why we've got letters and numbers in this address. MAC addresses are expressed in hexadecimal, which gives us the ability to express more values with the same number of bits. Theoretically, every single NIC in the world should have a totally unique MAC address, and the only way to do this is to express MAC addresses in hexadecimal.

MAC addresses are actually made up of two parts, so let's take another look at the one I showed you earlier.

af-14-b3-c2-14-45

The first half of that address (af-14-b3) is the Organizationally Unique Identifier. This particular OUI would belong to one and only one vendor, making it "organizationally unique". The second half of the address is a combination of hex characters that this particular vendor has not used before with this particular OUI, sometimes called the Device ID.

Breaking the example down into its two parts:
  • af-14-b3 is the OUI
  • c2-14-45 is the Device ID
In this way, the MAC address should be unique from any other MAC address in existence. (The use of hex means we can have 281,474,976,710,656 possible combinations.)

Note the highest hex value is f. If all values in a MAC address are set to f, that's the MAC broadcast address. Expressing a hex value in upper or lower case does not change the value, so both of the following are the same address. Watch out for any MAC address that contains a letter that comes after "F" in the alphabet - that's an invalid address. For example, both of the following MAC addresses are invalid. 11-22-33-44-55-hf Rf-12-34-45-56-67

MAC addresses can be expressed with hyphens, as we've seen so far in this chapter, or with colons. They can also be expressed in a format similar to IP addresses. To illustrate, all of the following MAC addresses are the same address and are all valid ways of expressing a MAC address.

aa-bb-cc-dd-ee-34 aa:bb:cc:dd:ee:34 aabb.ccdd.ee34

While we spend most of our time working with IP addresses, data can't be transmitted from one point to another without the right MAC addresses. In tomorrow's CCNA exam tutorial, we'll take a look at how switches build a table of MAC addresses and the actions a switch can take with incoming frames. See you then!

About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the CCNA exam with Chris Bryant!





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