IPv4 address shortage
Carrier-grade Network Address Translation (CGN or CGNAT) is Network Address Translation done by the Internet Service Provider (ISP). It is a short-term solution to provide IPv4 access where the ISP has more customers than public IP addresses. In this setup, multiple customers share a single public IPv4 address. CGN has the major drawback that it does not allow incoming connections at all, because the address translator does not know which customer an incoming connection was intended for. This makes it impossible or very difficult to host services such as websites or file servers. It also causes problems with some online games, online security cameras, and BitTorrent. Most land-line ISPs don't use CGN yet, but mobile providers often do. The address block 100.64.0.0/10 is reserved for CGN, although many ISPs improperly use 10.0.0.0/8 instead. ISPs that use CGN usually offer a public IPv4 address to customers that request one. They may offer this for free, charge a one-time fee, or charge a monthly or yearly fee. In the future they might not offer public IPv4 addresses or might charge a large fee for them, due to supply and demand.