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Chris Bryant, CCIE #12933
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! Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of over 100 free certification exam tutorials, including CCNA exam, Network +, Security+, and CCNP topics. You can also join his RSS feed and visit his blog, which is updated several times daily with new Cisco certification articles, free tutorials, and daily CCNA, Network+, Security+, A+, and CCNP certification exam practice questions! For a FREE copy of his latest e-books, “How To Pass The CCNA” and “How To Pass The CCNP”, visit the website and download your free copies. You can also get FREE CCNA and CCNP exam questions every day! Earn your CCNA certification with The Bryant Advantage! Article Directory: Article Dashboard Other articles from Computer Certification... |
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