The success of the Internet around the world has produced many benefits, but due to IP protocol stack limitations, the IP addresses exhaustion problem soon arose. Although it has been proposed several solutions to alleviate it, the problem still remains, specially in the emerging countries, which have a reduced pool of public IPv4 addresses. Classless Inter-Domain Routing (CIDR) scheme has partially solved this problem by introducing a mask to decide which bits belong to the host-ID. However, the mask has an inherent limitation since all the sizes must be a natural power of two. In this paper it is proposed a new routing scheme called Maskless Inter-Domain Routing (MIDR) which is able to define networks of any size. MIDR is totally compatible with other routing schemes, like CIDR or IPv6. Therefore, the Internet Service Providers (ISP) can test and use MIDR in their networks without interfering in their already established background traffic.