The devices surrounding us become smarter and can autonomously form a network without requiring our intervention. However, our needs can be even better accommodated when the networked devices cooperate and complement each other's capabilities. One of the initial steps towards achieving a cooperative platform of smart devices is the discovery of resources and capabilities within the network. Today's operational service discovery protocols carry simple text-based uniform resource identifiers that are not expressive enough. Machines cannot comprehend the meaning of a new service that is not in their knowledge base. In addition to being more expressive, service discovery protocols must compensate the diversity to improve cooperation between the devices that use different application protocols and operate on different communication interfaces. In this paper, we propose the Smart Discovery Protocol (SDP) which outperforms the operational service discovery protocols with three main features: (1) more expressive semantic representation of the services, (2) operating in the network layer to deal with diversity, and (3) unifying existing service discovery protocols. SDP represents services with ontologies as some recently proposed semantic service discovery protocols. It further enhances the success of semantic representations by creating a unified platform that can carry legacy discovery services. In this respect, the novelties of SDP are as follows: firstly, it operates in the network layer and consequently abstracts both the application layer and communication interfaces. Secondly, SDP unifies the legacy service discovery protocols by integrating their simple text-based service representations in one message. The underlying transport mechanism of SDP is designed as an add-on to the Neighbor Discovery Protocol (NDP) of the IPv6 standard. The metadata is carried in the payload of ICMPv6 packets. Simple text-based representations of other service discovery protocols are embedded in type-length-value options of NDP. Authenticity of the devices is ensured by the IPv6 Secure Neighbor Discovery protocol. Unlike previous semantic approaches on service discovery, we have implemented our protocol on real hardware. The results demonstrate the feasibility of carrying semantic representations of the services and integration of other service discovery protocols.