A lot of the time, on web forms, you will see some sort of structured field, like an IP address, or credit card number, or account number, broken up into a series of field boxes. You see this is in program GUIs as well.

On the surface it makes sense. Never throw away structure information. If you're parsing a human name, it may be impossible to parse it as well from a plain string compared to a set of boxes for first, last and middle names.

But this does not make sense if the string can always be reliably parsed, as is the case for IP addresses and account numbers and WEP keys and the rest. Using multiple boxes just means users can't cut and paste. And it's also hard to type unless you are ready to hit TAB at a point your mind wants to type something else. Some sites use javascript to auto-forward you to the next box when you've entered enough in one box, but it's never perfect and usually doesn't do backspace well.

Think about it. The multi box idea, expressed to extremes would have every form enter an e-mail address with a username box and a domain name box, with an @ printed between them. This would stop you from entering e-mail addresses without at signs. But fortunately nobody does it. We can always parse an E-mail and we don't want to subject people to the pains of typing it in a strange way.

Now I have to admit I've been tempted sometimes on international phone numbers, because parsing them is hard. The number of digits in the various components, be they area codes or exchanges, varies from region to region and I am not sure anybody has written a perfect parser. But nor do people want to enter phone numbers with tabs. And they want to cut and paste. Remember this when designing your next web form.