A process of elimination is the best approach to solving network issues.

When a network problem strikes, it isn’t always immediately obvious. It’s rare to get a clear message on the screen, and it’s easy to imagine that Facebook has simply crashed or a website is temporarily offline. A good first check is to open a new window in Safari and try pointing at google.com. The web requires the least complicated protocols of any of the services that run over the internet, and Google has a nice fast webpage that is always up. The front page itself could be loading from Safari’s internal cache, of course, so test your live connection to the internet by typing something random into the search box to force it to query the server.

If you can’t even reach google.com, the problem is almost certainly at your end of the internet.

If you get a page of results, then you at least have an internet connection. If you get a progress bar that goes nowhere, followed by a “Safari can’t find the server” page, you’ve got problems.

If you have a wireless connection to your broadband router, cast a glance up at the menu bar on your Mac. The “stripey slice of pizza” icon on the right-hand side should be black. If it’s grayed out, click it and see if you’re connected to the right network. Sometimes you’ll see your Wi-Fi network listed, but it won’t have a checkmark to show that it’s selected. This is because the network briefly disappeared from the radar and OS X didn’t automatically reconnect when it came back up. Just click it in the list to reconnect. If your Wi-Fi link looks okay but you still have no internet connection, it’s time to take a look at the router.

Has your Mac accidentally connected to the wrong wireless network?

Broadband modem/routers are little computers running an embedded operating system, and they can crash just like any other computer. If none of the lights on the front are flickering—either because they are all stuck on or they are all switched off—turn the modem off at the wall. Actually, do this even if your modem lights look normal—different modems use different light sequences to indicate problems and rebooting the modem is an easy and quick way to rule out a crash.

Light Show

When your modem starts up, it will begin with a power-on self-test, which is normally indicated by the power light flashing steadily. After about five seconds of this, the power light will change to steady. Next to light up will be the LAN lights for any wired Ethernet connections that are active, closely followed by the WLAN light, which should start blinking away. After that, there will be a pause as your modem logs in to your ISP, and then the light labeled Broadband, ADSL, or just DSL will come on. After that, there should be another pause before the internet light comes on to indicate that the router has been allocated an external IP address from the ISP.

Consult the manual for a breakdown of what the lights on the modem mean.

If all the lights come on in more or less this sequence, then you can be confident that the broadband modem is working fine. If any of the lights are yellow or red, or if the broadband or internet lights don’t come on at all, then it could be that your modem is damaged. This can happen if the modem isn’t ventilated and has overheated, or if the phone lines near your house are struck by lightning, sending a power surge to the modem. (This is why it’s a good idea to disconnect the phone line from the modem if you have a big thunderstorm nearby.) Alternatively, your ISP may be having technical problems at your exchange. Try calling their support hotline to check this.

Simplify, Simplify

Assuming that the modem restarts correctly but your network problems remain, the next step is to take as much complexity out of your network as you can. This will help you identify which link in the chain is failing. Does your iPod touch or iPad still connect through Wi-Fi? If so, it’s a sign that the problem lies with your Mac. If no Wi-Fi device can connect, try a wired connection.

Lots of things can cause Wi-Fi networks to suddenly break, but most of them boil down to either incompatibility with another network device or problems with wireless security. If you have recently changed your broadband provider, your wireless security key will have changed, but the new modem/router might also be using a different wireless security protocol. Some older network devices and computers have problems with the newer WPA2 protocol. Try temporarily turning security off and see if that makes a difference. It’s possible you might need to fall back to the older WEP protocol. This is less secure than WPA or WPA2, but it’s still better than nothing. Wireless security is only to protect yourself from hackers within physical wireless range of your network, so depending on your neighbors, you might not really need more than WEP.

DHCP automatically assigns IP addresses to your network. When it works, it’s very convenient.

Every device on your network must have a unique IP address. These take the form 192.168.1.n, where n is a number between 0 and 255. When you install a device, you can either choose a number for it yourself or you can let the router allocate one automatically, using a service called DHCP. Using DHCP for all devices is usually the easiest option, but some devices don’t support it or behave oddly with it. If you assign static IP addresses to some of your network devices, make sure that you restrict the address range that the DHCP server on your router can use, and assign static IP addresses that do not lie in this range.

WHAT FIREWALL? Test Your Digital Defenses!

While it’s theoretically true that you can never have too much security, it’s important to retain a sense of proportion. You are not the Pentagon, you do not have powerful enemies or valuable secrets. The internet is not full of black hats, targeting you for attack. What it is full of is people doing the cyber equivalent of walking down a row of parked cars, surreptitiously trying the door handles, in case one of them is unlocked.

A good firewall will make sure your car doors are locked. A really great one will make it look as if there isn’t even a car parked there at all. You can test how effective your firewall is by going to grc.com and following the links to the ShieldsUP page. Firewalls are a lot better than they used to be, and you’ll probably be pleasantly surprised by the results you get from the combination of the hardware firewall in your modem and the software firewall in OS X (System Preferences > Security).

Of course, a firewall is only part of your internet defenses. Make sure that you set a decent password for the admin account on your modem and you don’t leave your Mac’s guest account unsecured (System Preferences > Accounts), for example.

Frequency Issue: Sources of Interference

All the flavors of Wi-Fi other than 802.11a operate in the 2.4GHz portion of the radio spectrum. This is an unlicensed frequency, which is good because it’s much cheaper for manufacturers, but bad because it’s very crowded. Wi-Fi protocols, particularly wireless n, are designed to hop around the channels within their permissible frequency range, but even so, there are lots of things that can cause your wireless performance to degrade or cut out altogether. The most significant is other Wi-Fi equipment. Most routers are factory-configured with the same default channel and almost no one ever bothers to change this. If your neighbors have their router placed directly on the other side of the wall from you, this could be a problem. Also if you have a Time Capsule right next to your router, by default they will both be broadcasting different Wi-Fi signals and will interfere with each other. Turn Wi-Fi off on one of them.

If your router is too close to the microwave, your reception will be fried.

Other significant sources of interference are microwaves and video-streaming boxes. Reposition your router or change the Wi-Fi channel on the setup page, then check to see if that helped.