Carrier-grade NAT, also called large-scale NAT, is NAT done by the Internet service provider (ISP). This is used when the ISP has more customers than IP addresses. This is commonly used by cellular service providers on 3G and 4G networks, but it is not (yet) common on landline DSL or cable connections. The CIDR address block 100.64.0.0/10 is reserved for carrier-grade NAT, although many ISPs improperly use RFC 1918 private addresses, which are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.