May 23, 2002

By Karen Kenworthy


Let's see ... I have a telephone number, a fax number, and a cell phone number. I have a street address, two credit card numbers, a bank account number, and a mortgage loan number. Then there's my auto club membership number, my "Riders In The Sky" International Fan Club membership number, and the number on my library card. Add to that several email addresses, login names at several web sites and online services, and a few account numbers at "brick and mortar" stores I frequent.

Whew! That's nowhere near a complete list. But you get the idea. We humans love giving each other "IDs", names or numbers that identify us.

Our computers enjoy this sport too. From drive letters, to port numbers, to login names, computers love to assign IDs. Today, let's take a close look at two forms of computer ID -- one an old friend, IP Addresses, and the other new friend, the MAC Address ...

IP Addresses

We've talked about IP Addresses before. The letters "IP" stand for "Internet Protocol," and each computer connected to the Internet must have at least one. Think of these unique numbers as your computer's "membership" number, assigned when it joins the Internet. Without an IP address, your computer can't play on the 'Net.

Now you may be wondering, "What do IP addresses look like?" The answer depends on who you ask. True to their numerical nature, our computers see IP addresses as 32-bit binary numbers. For example, to them, the IP address of one of Microsoft's web servers looks like this:

  1100 1111 0010 1110 1110 0110 1101 1011

But we humans don't get along well with long strings of 1s and 0s. We prefer numbers with a wider variety of digits, preferably digits ranging from 0 to 9. These are what mathematicians call "decimal" numbers.

So you might think that humans would convert binary IP addresses into a large decimal number. If they did, the IP address shown above would look like this:


But if that doesn't look familiar, you're not alone. In fact, thanks to some clever network designers with time on their hands, human-friendly IP addresses actually look like this:

The "recipe" for creating this "dotted quad" form of an IP address is simple. First, divide the 32-bit IP into four equal parts, containing 8 bits each. Next, convert each of these 8-bit binary number into its decimal equivalent. These decimal numbers will fall between 0 and 255 (the smallest, and largest, possible values of an 8-bit binary number, respectively). Finally, add periods to taste. :)


I guess every parent has to answer this embarrassing question, sooner or later. "Mommy, Daddy, where do IP addresses come from?" Of course, you can give the simple, easy answer -- "Why, IP addresses come from ICANN (The Internet Corporation for Assigned Names and Numbers)" then quickly change the subject.

But while this reply might satisfy a young child's curiosity, you and I know the real answer is much more complicated ...

Unlike the early days of the Internet, today ICANN doesn't directly supply IP addresses to any computer or network. Instead, ICANN divided most of the available IP addresses into three large groups, and has given one group to each of three "Regional Internet Registries" (RIRs).

That doesn't sound too complicated, does it? For example, Europeans might get addresses from RIPE, or the "Reseaux IP Europeens." Asians could get an IP address from APNIC (the "Asia Pacific Network Information Centre"), while folks living North and South America would shop for their addresses at ARIN (the "American Registry for Internet Numbers").

But of course, things aren't that simple. Only the largest companies and Internet Service Providers (ISPs) obtain IP addresses directly from a RIR. The Registry's give away addresses a few million at a time. Those who need "only" a few thousand IP addresses, let alone folks who need only one or two, need not apply.

Large companies divide the IP addresses they've received among subsidiaries and their large networks. From there they eventually find their way to individual computers. Large ISP's allocate IP addresses to their customers, some of which are smaller ISPs. Eventually, these IP addresses, too, make their way to small networks and single computers.

But how? How do computer's discover the IP address they've been given from on high?

Some computers are assigned "static" IP addresses, addresses that don't change over time. Often these computers operate web sites, or provide other public services. Keeping their IP address constant makes life easier for the rest of the Internet world that must communicate with them. Normally, a system administrator "tells" each of these computers their IP address.

But most computers on the Internet have temporary, or "dynamic" IP addresses. Over time, these addresses can, and do, change. For example, if you connect to the Internet via a modem and a phone call, an IP address is assigned by your ISP for the duration of your connection. This assignment of the IP address (plus the exchange of your ISP account name and password) is handled by something Internet experts call Point-to-Point Protocol, or PPP.

Computers attached to a Local Area Network (LAN) learn of their IP addresses in a different way. In most cases, they contact a special computer on the network, known as a DHCP (Dynamic Host Configuration Protocol) server. Computers with "always on" connections to the Internet, such as cable modems or DSL lines, must contact a DHCP server too.

When contacted, this server loans, or "leases," IP addresses to other computers. The term of these leases lasts for a few hours, up to a few days. Once a lease has expired, the computer must contact the DHCP server again, to renew its lease or obtain a new IP address.

MAC Addresses

The sharp-eyed among us may be wondering, "How can a computer communicate with a DHCP server, to obtain an IP address?" After all, aren't IP addresses used when sending information between computers?

To find the answer, let's look at the special circuitry a computer uses when communicating over a LAN, or other broadband pathway such as a DSL line or cable modem. It's called a Network Interface Card (NIC), because it connects a computer to a network (your LAN, or the network of your ISP).

At one time almost all NICs resided on a separate circuit card, called a network adapter, hidden inside your computer. But today, a NIC may be built into the circuits found on a computer's motherboard. In the case of laptops, a NIC may even be inserted into a special opening called the PCMCIA or PC Card slot.

Wherever it's found, permanently stored within the NIC is a unique 48-bit binary number called the MAC (Media Access Control) Address. And it's by this MAC address that each NIC is known, within a local network.

Our electronic friends only appear to use IP addresses when sending messages across a LAN. Unseen, our computers translate each IP address into a corresponding MAC address, and use the MAC address to route each packet of data. And when talking to a DHCP server, they can even eliminate that conversion, using the server's MAC address directly.

To make sure every MAC addresses is unique, the first 24 bits of the address identify the NIC's manufacturer. Manufacturer's are assigned these numbers, called an Organizationally Unique Identifier (OUI), by the Institute of Electrical and Electronics Engineers (IEEE).

The remaining 24 bits of each MAC address are a serial number, assigned to a particular NIC when it is made. This allows each manufacturer to create a maximum of 16,777,215 (the largest possible 24-bit number) NICs. When that limit is reached, the manufacturer must reapply to the IEEE for another OUI.

This arrangement allows you to determine the manufacturer of a NIC, if you know the NIC's MAC address. Simply enter the first 24 bits of the MAC address (in hexadecimal notation) at this section of the IEEE's web site:

While there you can also apply for an OUI of your own. They're only $1,650. :)

To discover the MAC address of all NICs found in your computer, check out Karen's Computer Profiler at:

This program will also reveal the IP addresses assigned to your computer. And unlike an OUI, the Computer Profiler is free. :)

You might also want to try Karen's URL Discombobulator, a program that can disclose the IP addresses assigned to any remote computer with a domain name, such as Another Power Tool, Karen's WhoIs, can even tell you which RIR, and which company or ISP, was allocated that IP address. You'll find these two programs at:

And as always, if you prefer the convenience of a CD or want to support Karen's Power Tools, visit my CD home page at:

There you can order your own copy of Karen's CD, complete with the latest Computer Profiler, URL Discombobulator, and WhoIs. Your CD will also include the most recent versions of every Power Tool, plus three bonus Power Tools programs not available anywhere else. The CD even has all the back issues of my newsletters, and a special license that lets you use all the Power Tools at work!

As for me, I may spend some time listening to the "Always Drink Upstream From The Herd" CD by "Riders In The Sky." Their version of Gene Autry's hit "Take Me Back To My Boots And Saddle" can't be beat! If you're a secret buckaroo or buckarette, you might want to find a copy, kick off your boots, and have a listen too.

And if you see me riding the range, or cruising the 'net, be sure to wave and say "Hi!" After all, it's the Cowboy Way. :)

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