Why should I get IP addresses from my ISP rather than the RIPE NCC?

For one thing, the RIPE NCC cannot guarantee that any address space it allocates will be globally routable. This is because the filtering policies implemented by transit providers around the world are beyond the RIPE NCC's control. The best way to avoid filtering problems is to ensure that your network is numbered into the address range of a larger network (such as your upstream ISP), so that many addresses can be aggregated and announced globally as a single route. The shorter prefix of such a global route will increase its chances of being unaffected by the filtering policies of large transit providers.

It is also in the interests of the Internet community as a whole that the number of separate routes injected into the global routing tables is kept to a minimum. Encouraging organisations to obtain address space from upstream ISPs wherever possible is important to achieving this aim.