Will we be able to depend on the Internet in the event of a disaster? As with many questions the answer is, "It depends."
Although it's primarily operated by commericial interests now, the Internet -- at first called ARPANET -- was originally a Department of Defense project to create a decentralized national communications system that could survive a nuclear war. No one entity owns or controls the Internet. In fact, the Internet is really made up of an ever-changing collection of inter-connected networks, albeit one controlled by a relatively small number of network backbones operated by major carriers, connected via peering arrangements.
What this means is that taking down the entire Internet is difficult. One carrier or another may suffer significant outages but it is unlikely that all of them will simultaneously.
A more serious threat to the Internet would be a coordinated attack on the root domain name servers. The root DNS servers are at the top of the hierarchical DNS system, which translates host names (e.g., survivalpreps.blogspot.com) into the numerical IP addresses computers use to access other systems on the Internet. Thankfully, the root servers are well protected against hacker attack and are geographically dispersed.
Because the Internet is made up of multiple redundant systems, chances are that during a widespread disaster that large parts of it will remain available. There are several tools we can use to communicate during such an event:
Email allows us to send and recieve text messages along with attachments. It's a good idea to have one or more free webmail accounts that you can fall back on if your Internet service provider's mail servers are offline.
Instant messaging allows us to do real-time text messaging with others. Many IM clients also allow voice chat, so that with a speaker and microphone, you can carry on a voice conversation. The most popular IM programs are free and include AOL Instant Messenger (AIM), Yahoo! Messenger, and Windows Live Messenger.
A personal anecdote: On 9/11/01 I tried to reach several family members in and around New York City from my home in Philadelphia. Getting a telephone connection was largely impossible, all circuits were busy. However, I was able to get in touch with a couple of my cousins on Long Island via AIM. Even though the fall of the World Trade Center took out a Verizon telephone siwtch and a lot of Internet equipment, the Internet is designed to route traffic around outages, as long as another path is available. This worked on 9/11.
One advantage the IM clients have is low bandwidth requirments. Passing login information and plain text messages doesn't require much available network bandwidth. One disadvantage is that the major commercial IM clients require a central server to handle user logins and traffic passing. So, if the central server is offline the system is useless.
Skype is an exception to this. Primarily a voice messaging system, Skype also includes a text messaging function as well. But what makes Skype especially attractive for emergency communications is the fact that it is a peer-to-peer system. Instead of depending on a central controlling server, Skype works in a decentralized manner, which makes it more resilient in the event of a widespread disaster.
SypeOut is a cool feature that allows you to make Skype-to-telephone calls. It allows you to make a call from your computer to a regular telephone. Through the end of 2006, SkypeOut is free when both the caller and callee are within the USA or Canada. SkypeIn is the reverse -- assigning a telephone to your Skype installation on your computer so that you can receive calls from regular phones.
Voice quality for Skype is usually quite good. Having a good set of speaker and mic, or a headset will greatly improve call quality.
One final feature of Skype makes it especially attactive: all Skype-to-Skype communications are encrypted, allowing you to maintain privacy.
Blogs can be used as a way to provide updates on a situation to many people at once. During the immediate aftermath of Hurrican Katrina, employees of a New Orleans-based hosting company directNIC maintained the "Interdictor" blog so that people on the outside could get a participant's view of the situation, including webcam feeds. Blogger and Livejournal provide free blog hosting services.
Obviously, if your area is heavily impacted by a disaster, immediate Internet connectivity may not be easily available. For example, power may be out for an extended period. Even so, ISPs usually have their equipment on backup battery and generator power. If telephone or cable lines remain intact, you may be able to get online if you have a charged laptop and a dialup modem, or a cable or DSL modem on an uninterruptable power supply (battery backup).
Still, like anything man made, Internet access in an emergency is not a given. Make it one of the tools in your toolbox and you'll be better prepared to get safely through a disaster.