Mac OS X: Troubleshooting a cable modem, DSL, or LAN Internet connection

Symptoms

Mac OS X includes all the applications you need to access and use the Internet right away. But if your Internet connection doesn't seem to work, the following steps can get you back online. Tip: If you connect via PPPoE, follow the steps for PPPoE connections instead

Resolution

If you're on a local area network



Contact your network administrator instead of an Internet service provider (ISP). Information specific to modems may not apply to LAN users. LAN users may have hubs, switches, routers, or connection pods that cable and DSL users may not have.



If you use AirPort



Simplify the troubleshooting procedure by temporarily removing the base station from your network. Set up one computer to connect directly to your DSL or cable modem using the steps below. After re-establishing service to the computer, see the AirPort section below. Alternatively, troubleshoot from a more AirPort-specific perspective with the AirPort Troubleshooting Guide.



Check all cables and power supplies



Make sure all the cables related to the modem are plugged in and firmly inserted. This includes the modem's power cord, the cable from the modem to the computer, and the cable from the modem to the outlet. To be sure a connector is properly inserted, disconnect and reconnect it. Check to see if the cables have been cut, chewed, or otherwise damaged.



If you have any hubs or routers, check their cables and power supplies. If you have more than one power supply available for a type of hub or router, try swapping the power supplies. A defective power supply may affect network traffic.



Reset/power cycle modem hardware



Cycle the power on your DSL or cable modem by turning it off for a few minutes then turning it back on. Some ISPs recommend you unplug the modem's power cord. This is required if the modem does not have a power button. If the modem has a reset button, you can try it before or after cycling the power.



Check indicator lights on the modem



First, check to see if the DSL or cable modem has a "standby" light or button. Some popular modems may not connect to the Internet when in standby mode. If applicable, be sure the modem is not in standby mode.



Other indicator lights can provide useful information about cabling connections. A light named "Cable" or "Online," for example, indicates whether the ISP's network is available. A "PC" or "link" light indicates whether the hardware connection between the computer and modem is good. A positive link light almost always indicates that the Ethernet ports on each device are working properly, and thus a positive link light makes it very unlikely that an Ethernet port failure is causing the issue.



Tip: If the lights on your modem vary only between on and off and do not change color, then a "positive" light is one that is on. If the lights on your modem change color, a green light is more likely to mean positive than yellow or red.



If your link light is positive, skip to the Check Network preferences settings section. If there is no link, try modem and Ethernet hardware troubleshooting.



Broadband modem and Ethernet hardware troubleshooting



If the link light is off after you checked your cables and cycled the power, you need to re-establish the link before continuing. If your computer came with an Apple Hardware Test CD, use it to check for any issues with your hardware. If nothing is found, contact your Internet service provider. They may be able to remotely test your modem to see if it has failed. If nothing is found with the modem, replace the Ethernet cable, since it is the most easily replaced component.



If replacing the Ethernet cable does not establish the link, you will need a third Ethernet device to determine whether the computer or modem is failing to link. The third device could be a simple ("dumb") Ethernet hub with link lights (an inexpensive 10BASE-T hub found at any computer store will do), an Ethernet switch with link lights, another computer, a router, or a wireless base station.



When using a hub or switch to test, simply connect both the computer and modem to the hub or switch, then check the link lights on the hub or switch to see which of the other devices is not connecting. If you use another computer (Mac or PC) to test, check to see if the other computer has an Ethernet link light (many do not). If there is no link light on the computer, set up a file sharing connection to verify connectivity. If you use a router or base station to verify connectivity, connect as instructed in the device's documentation. If the router or base station has link lights, you may use the simple link light test.



If you are not able to use this type of test, do whichever of the following is more convenient for you: either exchange your modem with your Internet service provider or take the computer somewhere else where you make sure it works, for example, a neighbor, school, work, and so forth.



Be sure that your software settings are correct, as described in the Check Network preferences settings section below.



If you still cannot connect, you may have a DNS issue. Your Internet service provider's DNS service may not be working, or you did not configure it correctly.



If you still cannot connect and your cable modem link light is positive, but you haven't replaced the Ethernet cable yet, substitute a new or known-good Ethernet cable for the one you have been using.





Check Network preferences settings



Use Network preferences to verify your TCP/IP settings and that Built-in Ethernet is your primary network interface. Follow these steps:

1. From the Apple menu, choose System Preferences.
2. From the View menu, choose Network.
3. Choose Active Network Ports from the Show pop-up menu ("Advanced" from the "Configure" pop-up menu prior to Mac OS X 10.1).
4. Be sure that the checkbox for "Built-in Ethernet" is selected.
5. Drag Built-in Ethernet to the top of the Port Configurations list to make it the preferred, or primary, network interface.
6. Choose Built-in Ethernet from the Show pop-up menu.
7. Click TCP/IP. Follow your ISP's instructions for setting up this pane. For more information, see "Mac OS X: How to Connect to the Internet With Cable or DSL".
8. If you have made any changes, click Apply Now.

If you still cannot connect, you may have a DNS issue. Your Internet service provider's DNS service may not be working, or you did not configure it correctly.



If you still cannot connect and your cable modem link light is positive, but you haven't replaced the Ethernet cable yet, substitute a new or known-good Ethernet cable for the one you have been using.



Sharing the connection



If you have two or more computers trying to share an Internet connection, make sure your network is set up properly. You need to know if your ISP provides only one IP address or if it provides multiple IP addresses. If only one IP address is used, you must have a network router capable of sharing the connection, also known as network address translation (NAT) or "IP masquerading." Check the documentation that came with your router for setup information. Check with the person who set up your network, or with the router manufacturer for assistance.



An AirPort Base Station can share a connection with wireless clients.



Additional steps for AirPort

1. After successfully connecting to the Internet with the computer, disconnect the modem from the computer.
2. Unplug power cords for both the modem and the base station.
3. Connect the base station to the modem.
4. Plug the modem back in first, then the base station, and wait for the base station's indicator lights to cycle through their startup routine.
5. Start up a computer, and try to connect to the Internet using the base station. If you cannot, use the AirPort Setup Assistant (/Applications/Utilities/) to copy your computer's network setting to the base station. If you have an advanced network setup, you may need to use AirPort Admin Utility instead.

Get help from your ISP



If you have not resolved the issue with the steps above, contact your Internet service provider.



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