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URNs are a special kind of URI that are less commonly encountered, but look similar to URLs (and begin with the characters "urn:"). URNs are different from URLs primarily because they tend to rely on some other mechanism to exert manage control over them.
Persistence
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Instead, use an entityID that describes the service itself. For example, if the School of Engineering at Example University is protecting their Blackboard installation, a reasonable entityID for the SP might be https://engineering.example.edu/blackboard/shibbolethsp.
Resolution
As mentioned earlier, whether an entityID can actually be resolved into something is generally a secondary issue. SAML V2.0 defines a fairly obvious way of obtaining metadata about a given entity by resolving an entityID URL (see section 4.1 of the SAML Metadata Specification).
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If you wish to provide a resolvable document, please note the warning in t he the SAML V2.0 Deployment Profile for Federation Interoperability: "automatic generation of metadata has a strong tendency to undermine the correct functioning of peer deployments in the face of key rollover or changes to endpoints or other software features because it tends to change too suddenly to accommodate a graceful transition between states"
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