The first time I saw a URL shortening service, my internal alarms went off.

These services, such as tinyurl.com and bit.ly allow you to shorten the number of characters needed when you give somebody a Web address. For instance, if I want to email you the direct link to this article, I can send you the complete article, or I can give you a shortened version through one of these services to make it easier to type. My recent column about Carnegie Mellon's method of turning technology into revenue becomes 13 characters using bit.ly, instead of the 44 characters to directly go to that page using the Post-Gazette's page name.

But there's a danger in these shortened addresses. They can conceal websites that infect your computers with malware -- dangerous software of various sorts. According to Symantec's May 2011 MessageLabs Intelligence Report, it's even worse, as spammers are now establishing their own URL shortening services because they don't want to be blocked by the legitimate services.

With the rise of social services like Twitter that require brevity in what you type, it's likely to become an even bigger problem.

In some ways, it traces itself to the dawn of the plain-language Web itself. Interested business people and domain squatters rapidly registered most of the reasonable .com domains with short names. MLB.com was originally the domain of law firm Morgan Lewis and Bockius; but later became the domain for Major League Baseball. Go.com was registered by an Internet entrepreneur and later purchased by Disney. O.com is now owned by Overstock.com.

The common wisdom was to go short because most people wouldn't type in long names. That has changed over the years. Most people are willing to type longer names -- especially if it doesn't mean having to remember some strange set of characters.

In Twitter, though, that's no longer the case. You're limited to 140 characters. So that 44 characters in my CMU article (plus the seven character http:// that you need to include so it will be clickable after you type it) takes up more than one-third of the allowable typing space.

That's where bit.ly and tinyrurl.com come in. If you copy the page address into a field at those websites, they give you a different shorter name that you can use instead, making it easier to fit your entire message into your tweet on Twitter. I use bit.ly a lot for Twitter. LinkedIn, The New York Times and most recently, Twitter itself have started providing URL shortening. As of last week, if you type in a long Internet page address, Twitter will automatically shorten it for you.

But the person who clicks on it doesn't know the real domain in the destination address -- which normally is a hint as to whether the domain is legitimate or not. If you're expecting a CNN page, and it doesn't start with www.cnn.com, you might decide not to click on the link -- a good habit to help keep malware-free.

Unfortunately, now many people are putting shortened links into their email messages, making the problem more difficult to avoid. I won't click on these shortened links in email messages; and people get mad at me for refusing. But it's not worth taking the chance of infection, just because the person on the other side hasn't pasted in the full address.

It's harder to avoid clicking shortened domains due to the Twitter factor -- but not impossible. And there's no reason to ever click on one in an email message.

First Published: June 20, 2011, 7:15 p.m.