Every network card manufactured has a unique media access control (MAC) address number. This MAC address is what uniquely identifies you from the millions of other people connected to the Internet. MAC addresses are 48 bit alphanumeric addresses, so trillions of possibilities. The MAC address is assigned to a network interface card (NIC), which is then assigned an IP address. The IP address and MAC address work together to communicate on your network or on the Internet. Every web server or computer connected to the Internet has a network card with an assigned MAC address.
The ping command is a diagnostic tool that helps you identify IP addresses and host names connected to a network. You typically use it to identify if a remote computer is connected to the network. You can also use it to ping an IP address and get its fully qualified domain name.
You can’t directly ping a MAC address, but you can use the arp command to ping all devices connected to your own computer. This type of trick works for any device with a network card. For instance, you might want to find out how many printers are connected to a switch. You can telnet to that switch and run the arp command to see a list of devices (including printers) connected to that specific switch.
Finding the IP Address for a Device
The easiest way to practice with the ping, MAC and arp command is with your own computer. You probably have at least one device connected to your computer that you can view. You first need your computer’s IP address. You can do this by opening a command line and typing “ipconfig /all” in the command line utility. You might have several network adapters registered, so find the IP address for your main network card in the list.
After you find the IP address, you can run arp on your own computer. Type “arp –a ipaddress” where ipaddress is your IP address. The “-a” slash tells the arp command to list the arp cache on your computer. This lists all of the network devices connected to your computer, and the list also includes MAC addresses. MAC address is listed as the “physical address.” MAC addresses are the physical addresses, because they are static and assigned to your network card hardware.
The IP address is listed as the “Internet address.” You’ll notice that you have several IP addresses listed. Any hardware connected to your computer is listed, which could be a printer, router or a virtual device. This is one way to find IP addresses of servers or printers if you don’t know them by heart or they aren’t listed anywhere by your network administrator.
You can also telnet to devices to get a list of IP and MAC addresses connected to remote devices. This is helpful if you need the MAC address for a remote computer but it isn’t connected to your computer in any way.
By default, newer Windows versions don’t have telnet installed, but you can install it as an addon or install a third party telnet program to connect to your device. Telnet is a way to connect to a remote device and run commands on that device. For instance, if your router or switch supports telnet, you telnet to the IP address. When you connect to the remote device, you would then run the same arp command. If you are located on the remote machine through a program such as telnet, you can just type “arp –a” to get a list of the connected devices.
You can also use the ping command to then get the remote computer’s fully qualified domain name. Using the command line, you can type “ping –a” to get the full name of the remote computer. If you notice, you can build from knowing nothing of a remote computer to getting its MAC address, IP address and remote computer name. This is the advantage of using these small console applications available with each operating system.
If you want to delete or add arp addresses to a computer, the arp command also has a “d” and an “a” command line switch. These two switches are beneficial if you want to control the arp cache after you figure out the MAC address for a specific device. For instance, you might want to clear the arp cache for a printer or add it to a router.
To add or delete an arp entry, you simple type “arp –a” with the IP address or use “arp –d” to delete an entry.
The arp and ping commands are powerful diagnostic tools, and they are available in the Linux or Windows operating systems. You’ll need to know these commands to properly work with any network in the corporate world. They are more for simple diagnostics if you have a home network. When you support a large network, you’ll need these tools to find network devices, figure out their MAC addresses and determine if these devices are connected to the network. For instance, if you can’t ping an IP address, it could mean that the device’s network card isn’t working or it is just not powered on. This is very useful when you are located in one office and the remote machine is in another.