This is an exercise for code newbies. It actually doesn't require any programming, just a little bit of calculation:
Consider a service such as bit.ly that generate short Urls
Assume that the generated key is always 7 alphanumeric characters long (using both upper case and lower case letters)
Suppose people around the world are generating short Urls at a rate of 1000 short Urls every second.
Question: How long takes until bit.ly runs out of unique sequences for the short Urls?