An apparatus comprising means for performing: - receiving, by a network repository function, a request from a network function, characterized in that the request comprises a string associated with an instance identity of the network function and indicates a type of the instance identity of the network function; - determining, by the network repository function, the type of the instance identity of the network function from a set of instance identity types based on the request; - determining, by the network repository function, the instance identity of the network function based on the string associated with the instance identity of the network function and the type of the instance identity of the network function; and - transmitting, by the network repository function, a response to the network function, wherein the response depends on whether the instance identity of the network function was found in a list of network function instances registered at the network repository function;. wherein the set of instance identity types comprises an identity type for service mesh technologies, and wherein the identity type for service mesh technologies comprises a Secure Production Identity Framework for Everyone, SPIFFE, identity type, wherein the network function includes an NFInstanceID of the network function in the format of UUID for backward compatibility or in the format of SPIFFE ID, wherein the SPIFFE ID covers trust domain and locality, UUIDv4/v5, NF Type, and service names as part of the SPIFFE workload identity in SPIFFE Verifiable Identity Document, SVID, wherein a certificate of the network function includes the assigned NFInstanceID of the network function or multiple NFInstanceIDs of the network function if applicable for a target deployment architecture, and wherein the network function instance includes multiple SPIFFE IDs. - receiving, by a network repository function, a request from a network function, characterized in that the request comprises a string associated with an instance identity of the network function and indicates a type of the instance identity of the network function; - determining, by the network repository function, the type of the instance identity of the network function from a set of instance identity types based on the request; - determining, by the network repository function, the instance identity of the network function based on the string associated with the instance identity of the network function and the type of the instance identity of the network function; and - transmitting, by the network repository function, a response to the network function, wherein the response depends on whether the instance identity of the network function was found in a list of network function instances registered at the network repository function;. wherein the set of instance identity types comprises an identity type for service mesh technologies, and wherein the identity type for service mesh technologies comprises a Secure Production Identity Framework for Everyone, SPIFFE, identity type, wherein the network function includes an NFInstanceID of the network function in the format of UUID for backward compatibility or in the format of SPIFFE ID, wherein the SPIFFE ID covers trust domain and locality, UUIDv4/v5, NF Type, and service names as part of the SPIFFE workload identity in SPIFFE Verifiable Identity Document, SVID, wherein a certificate of the network function includes the assigned NFInstanceID of the network function or multiple NFInstanceIDs of the network function if applicable for a target deployment architecture, and wherein the network function instance includes multiple SPIFFE IDs. wherein the set of instance identity types comprises an identity type for service mesh technologies, and wherein the identity type for service mesh technologies comprises a Secure Production Identity Framework for Everyone, SPIFFE, identity type, wherein the network function includes an NFInstanceID of the network function in the format of UUID for backward compatibility or in the format of SPIFFE ID, wherein the SPIFFE ID covers trust domain and locality, UUIDv4/v5, NF Type, and service names as part of the SPIFFE workload identity in SPIFFE Verifiable Identity Document, SVID, wherein a certificate of the network function includes the assigned NFInstanceID of the network function or multiple NFInstanceIDs of the network function if applicable for a target deployment architecture, and wherein the network function instance includes multiple SPIFFE IDs.