Cable Modem Troubleshooting Tips

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MAC addresses

MAC = Medium Access Control.

MAC addresses are used as the source and destination addresses for data packets on a LAN. A MAC address is written as 12 hexadecimal characters (0-9, A-F). The grouping of those 12 characters into pairs separated by colons (00:20:40:70:F4:84) or hyphens (00-20-40-70-F4-84) is purely conventional, and is not related to function. Every network device in the world has a unique MAC address. The first 6 characters of the MAC address are unique to the manufacturer of the device. Allocation of MAC addresses is administered by the IEEE in the USA.

For some of the procedures described elsewhere on this site it is necessary for the user to discover various MAC addresses: the instructions for finding the following MAC addresses are listed on this page:

Finding the MAC address of the PC's network interface; Updated 2002 Aug 31
Finding the HFC MAC address of the cable modem;
Finding the MAC address of any other PC on the LAN; Updated 2002 Aug 31
Finding the MAC address of the WAN port of a router;
Cloning a MAC address into the WAN port of a router.

Finding the MAC address of a PC's network interface

Each network interface in your PC will have a different MAC address. Some cable ISPs require you to register with them the MAC address of the network interface of the PC/device that is connected to the cable modem. To discover the MAC address of a PC network interface, proceed as follows.

First:

If you are using a USB connection between the PC and cable modem, you must already have installed the USB drivers, and connected the USB cable between the cable modem and the PC.
If you are using an ethernet connection, you must have connected the ethernet cable between the PC and the cable modem.
Ensure the cable modem is powered up and operational.

Win9x/ME: click Start, click Run, enter the command winipcfg. Use the pull-down menu to select the network interface for the cable modem (rather than any PPP dial-up). Look for the line Adapter Address.
Windows (any version except Win95): open a command window, type the command ipconfig /all. Look for the output section related to the network interface for the cable modem, and look for the line Physical Address:

2 Ethernet adapter : 

Description . . . . . . . . : Intel DC21041 PCI Ethernet Adapter
Physical Address. . . . . . : 00-60-1D-F6-7E-5C
DHCP Enabled. . . . . . . . : Yes
IP Address. . . . . . . . . : 213.107.117.246
Subnet Mask . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . : 213.107.117.254
DHCP Server . . . . . . . . : 62.253.128.3
Primary WINS Server . . . . :
Secondary WINS Server . . . :
Lease Obtained. . . . . . . : 07 26 01 21:22:01
Lease Expires . . . . . . . :

WinXP:

Open control panel Network Connections.
Select the icon for the cable interface, right-click it, and select Status.

Click tab Support.
Click button Details.

Look for the line Physical Address.
Click Close.

Click Close.

Close the control panel.

Apple Mac OS 8.x/9.x:

Pull down the Apple menu, select Control Panels.
Open the control panel TCP/IP.
If you cannot see a button labelled Info:

Pull down the Edit menu and select User Mode...
Click Advanced.
Click OK.

Click button Info.

Look for the line Hardware address.
Click OK.

Close the control panel.

Apple Mac OS X: one of the following:

Pull down the Apple menu, select System Preferences, click Network, in the pull-down Show: select the interface required, look for the line Ethernet Address or Airport ID.
Open a Terminal window, type the command ifconfig en0 (where en0 is the interface in use), look for the line ether.

Finding the HFC MAC Address of the cable modem

HFC = Hybrid Fibre-Coax. This abbreviation refers to the port of the cable modem that connects to the CATV coax cable. Some cable ISPs require you to register with them the HFC MAC address of your cable modem.

The HFC MAC address of the cable modem is stamped on the manufacturer's label on the back or underside of the cable modem, or on the label of the box in which the cable modem was sold.

**HFC MAC address identification**
Cable Modem model	Identifying label	Typical address range
Motorola Surfboard 3100	HFC MAC ID:	00:20:40:xx:xx:xx
Motorola Surfboard 4100	HFC MAC ID:	00:20:40:xx:xx:xx 00:04:BD:xx:xx:xx 00:08:0E:xx:xx:xx
3Com CMX	EA=	00:50:DA:xx:xx:xx 00:50:04:xx:xx:xx 00:01:03:xx:xx:xx 00:10:4B:xx:xx:xx
3Com Tailfin	EA=	00:50:DA:xx:xx:xx 00:01:03:xx:xx:xx 00:04:75:xx:xx:xx
Terayon TeraJet	MAC Addr	00:E0:6F:xx:xx:xx
Pace digital STB		00:50:94:xx:xx:xx
ntl:home 100/120 (Ambit)	under barcode	00:D0:59:xx.xx.xx

It is not possible to discover the HFC MAC address of the cable modem by use of any commands such as winipcfg or ipconfig on the PC. However, if the cable modem offers diagnostics via a web page, the HFC MAC address might be listed there.

Cable modems might have multiple MAC addresses, one for the HFC side (cable TV coax socket), and one for the CPE side (Customer Premises Equipment, the ethernet socket). USB-capable cable modems also have two further MAC addresses: one for the cable modem USB socket, and another for the emulated network interface in the USB driver in the PC. Although it is possible to discover the CPE MAC address(es) of the cable modem, there is no application or procedure that requires them.

Finding the MAC address of any other PC/device on the LAN

This procedure only works for PCs/devices in the same same IP sub-net as the PC doing the discovery.

For Windows:

Open a command window.
Type the command ping xxx.xxx.xxx.xxx, where xxx.xxx.xxx.xxx is the IP address of the PC/device for which you want to know the MAC address. Ensure that you get at least one reply.
Type the command arp -a
In the column Internet Address, look for the sought IP address, and then in the same line, read the corresponding MAC address from the column Physical Address.

For Apple Mac OS 8.x/9.x, the utility IPNetTuner can display the ARP cache of MAC addresses.

For Apple Mac OS X:

Open a Terminal window.
Type the command ping xxx.xxx.xxx.xxx where xxx.xxx.xxx.xxx is the IP address of the PC/device for which you want to learn the MAC address. Ensure that you get at least one reply, and cancel the ping commadn with Ctrl-C.
Type the command arp -a
Each line of output gives the equivalence between an IP address and a MAC address: look for the line with your sought IP address in it.

Finding the MAC address of the WAN port of a router

If you have a NAT router between your PC(s) and the cable modem, and if your cable ISP normally requires you to register the MAC address of your PC's network card, then you will instead need to register the MAC address of the WAN port of your NAT router.

To discover the MAC address of the WAN port of your router, please consult the documentation for your router. For instance, the procedure for the Linksys BEFSR41 is:

Send your web browser to the Linksys configuration at http://192.168.1.1/ and quote the password for access;
Click tab Status;
The WAN MAC address is one of those displayed.

Cloning a MAC address into the WAN port of a router

If you want to insert a NAT router between your PC and the cable modem, you might want to avoid changing the client MAC address that the cable modem connects to, for either of these reasons:

to avoid having to register a new client MAC address with your ISP (if your ISP is one which requires registration of the MAC address of the connected device);
to avoid problems with the ISP's DHCP system not immediately issuing a new IP lease when the client MAC address changes (see Swapping computers on the cable modem).

Many NAT routers intended for the domestic market have the ability to clone the PC's MAC address into the MAC address of the WAN port. For best effect, you should do the cloning before you first connect the router to the cable modem. To discover how to clone, please consult the documentation for your router. For instance, the procedure for the Linksys BEFSR41 is:

Discover the MAC address of the PC that you want to clone as in Finding the MAC address of a PC's network interface above.
Send your web browser to the Linksys configuration at http://192.168.1.1/ and quote the password for access;
Click tab Advanced;
Click tab MAC Addr clone;
Enter the PC's MAC address, two characters in each of the six boxes.
Click Apply.

If the router had already been connected to a cable modem, then after cloning a new MAC address into the WAN port, you must perform the procedures of Swapping computers on the cable modem.

With the cloned MAC address in the router's WAN port, then as far as the cable modem is concerned, it is talking to the original PC.

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