Uniform Resource Identifiers (URIs) are very popular among non-technical people, to identify services and users in Internet, because they hide the underlying complexity of IP addresses and port numbers with a simple syntax. However, as currently defined, URIs are not extensible enough to support other name resolution mechanisms than the Domain Name System (DNS), nor newer transport protocols. This paper defines a backward-compatible syntax for URIs, that allows the location part of any URI to be defined with a URI itself. This nested URI syntax is more flexible, as it makes it possible for current applications to employ dynamic Service Discovery protocols, and to support multiple transport protocols.