So basically because when the guys who created the internet didn’t think it would get this big, they coded with only 32 bits of address space. What this means is that you get 400 million unique memory addresses (in this case IP addresses) [32 bits ^ 2].
There is apparently 232 million addresses left, which will last about another year.
They have already created a solution, basically adding more bits to the IP addresses, but it is very expensive for companies to upgrade their systems to accommodate the new bits.
So don’t start freaking out, this isn’t Y2k all over again, but stay tuned for more!