All the examples provided in this section are hypothetical. Real world naming schemes will most likely look different.
Using the MRN identifier scheme, a vessel with an IMO number of 9743368 could be identified as follows:
urn:mrn:imo:imo-number:9743368
The governing organization that assigns IMO numbers is the International Maritime Organization (IMO). IMO may delegate the actual assignment of numbers to another organization, but it is still the organization that determines that an IMO number is unique. Within the context of maritime resource names, the organization ID (OID) refers to the organization that governs the syntax and rules of a particular resource type. In the example above, the organization ID is "imo".
Each organization further divides the organization-specific string (OSS), which is the part following "imo", into two parts.
The first part is an organization-specific namespace ID (OSNID), which is a unique identifier within the governing organization for a particular type of resource. In this example, we have used "imo- number," but this could just as well have been "imonumber" or even simply "number".
The second part is the organization-specific namespace string (OSNS). This is the only part that differs for resources of the same type; in this case it is "9743368". The organization-specific namespace string is, as the name implies, specific to a particular combination of OID and OSNID. In this case, the organization-specific namespace string is always a 7-digit IMO number.
Another way to identify the same vessel might be to use its MMSI number. Here the identifier could look like this:
urn:mrn:itu:mmsi:538070999
In this case ITU is the governing body because MMSI numbers are based on ITU recommendation M.585. It is possible that national bodies might do the actual assignment of MMSI numbers, but ITU is the governing body for the standardization of MMSI numbers.
These two examples show how multiple identities can identify the same entity; in this case, the same vessel can be identified by either an IMO number or MMSI number. This is similar to how an individual might be identified either by a driver license number or a social security ID. Note that some parameters that are frequently used for identification, such as human names, do not generally qualify as identifiers because they are not guaranteed to be unique. A single identifier must refer to one and only one entity.
URNs range from very coarse-grained to very fine-grained. For example, a container ship might be identified by one of the two previous URNs. The containers aboard the ship might be identified with an URN adapting the ISO 6346 identifier scheme for container ids.
urn:mrn:bic:container-id:csqu3054383
Finally, individual items in a single container might be identified by another URN scheme. It might even be possible to integrate with URNs defined outside of the urn:mrn namespace. For example, all items in a container might be identified by an electronic product code ([@!RFC5134]). In other words, the use of URNs as identifiers is not limited to those defined within this document. In the future, other non-maritime sectors might even adopt similar naming schemes based on URNs to facilitate easier integration across sector boundaries.
As mentioned earlier, an identifier does not need to be a physical object; it can be a virtual item such as an electronic document. For example, IMO might decide that all of their documents should use a "publications" prefix. The publication "IMO SOLAS Consolidated Spanish Edition, 2014 IF110S" might be referred to as:
urn:mrn:imo:publications:if110s
On the other hand, an organization such as IALA might decide that their publications should follow another format where the category of the publication is included in the identifier. For example, a recommendation could be:
urn:mrn:iala:publications:recommendation:e-nav-140
The identifier of a guideline might be written as:
urn:mrn:iala:publications:guideline:synchronisation-of-lights-1069
As can be seen from the previous example, the organization-specific namespace string can be split into multiple hierarchies. The governing organization can decide how it wants to structure its identifiers.
Another example of identifiers with multiple hierarchies could be seen in an identifier scheme for lights and buoys. Here IALA could choose to let the OSNS consist of <CountryCode>:<National Identifier>. For example:
urn:mrn:iala:aton:us:1234x5
There are no requirements that organizations be permanent entities. For example, the European STM Validation Project could choose to use "stm" as its organization ID. A voyage ID in this project might look like this:
urn:mrn:stm:voyage:id:xcus231230
Within the project, the group may use "xcus231230" to refer to a voyage plan. However, the full URN can be used when working with external systems or other projects, in case another type of identifier is also used for a particular voyage.
As can be seen from all of these examples, the scheme is highly adaptable. Each organization can choose its own layout for a specific type of identifier. It is easy to fit existing identifiers into the naming scheme, and it provides good context information about the type of the identifier, unlike something simple such as a random UUID.