CCNA Exam Tutorial: 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 lets 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 cant be transmitted from one point to another without the right MAC addresses. In tomorrows CCNA exam tutorial, well 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 (thebryantadvantage digital cameras ), home of free CCNA and CCNP tutorials, and The Ultimate CCNA and CCNP Study Packages.

Octagon House Floor Plan | The Curiousity Of An Eight Sided House

Even after 150 years, the octagon house floor plan remains a curiousity of architectural design. The original architect, John Richards, first designed the octagon house just before the Civil War. Whether he was in competition with his fellow land owners and town leaders or was merely trying to impress Eliza Forbes, his heiress wife. The impressive structure he left behind still draws fascinated visitors to the elegant eight sided mansion in Watertown, Wisconsin every day.

The original octagon house floor plan, built in 1854, is a fantastic 5 story brick building with 57 rooms. It also contains forerunners of modern conveniences such as running water, central heating and ventilation systems. A 40-foot-high cantilevered staircase also graces the interior. The story behind the octagon house was that Mr. Richards made a promise to his wife, Eliza, to build her the finest home in the Midwest.

John Richards was most likely influenced by fellow designer, Orson Fowler, who was a leading spokesperson for the construction of eight sided houses. At that time, the octagon house was touted as being an efficient, economic and beautifully esthetic housing design that dated back to the ancient Roman architects.

There were at least 36 homes constructed from the octagon house floor plan between 1844 and 1905. All of these homes were located in Wisconsin. Octagonal houses continued to be built through the Civil War. After that time period, most octagonal houses were either torn down or remodeled until they lost their original shape.

John Richards's octagonal house was given to the Watertown historical society in 1938. The house was in nearly mint condition and maintained with no electricity, no modern plumbing and not telephone - just as it had been when Mr. Richards and his wife lived there in the 1850s.

Tours of the Octagon house can be made daily May through October and directional signs are posted throughout Watertown. Summer hours are from 10 a.m. to 4 p.m. and 11 a.m. to 3 p.m. after Labor Day. Admission is $7 for adults; $6 for AAA members; $6 for children ages 6 through 12 and children under 6 are free.

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