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Some serial numbering systems include built in error-checking to deal with copying errors, using a check digit.

The second way of providing unique identifiers is using a name or code.

These are typically still assigned by a central authority, sometimes known as a registration agency, but they are constructed in different ways.

Encoding information about geography and hierarchy within codes can be useful. It can make them easier to validate.

It also mean you can also manipulate them,

But encoding lots of information in identifiers also has its downsides. The main one being dealing with changes to administrative areas that mean the hierarchy has changed. Do you reassign all the identifiers?

some identifier systems look at reducing the burden on that central authority.

federated assignment. This is where the registration agency shares the work of assigning identifiers with other organisations.

Another approach to reducing dependence on, and coordination with a single registration agency, is to use what I’ll call “local assignment“.

A simplistic approach to local assignment is “block allocation“: handing out blocks of pregenerated identifiers to organisations which can locally assign them.

Here the registration agency still generates the identifiers, but the assignment of identifier to “thing” is done locally.

A more common approach is to use “prefix allocation“. In this approach the registration agency assigns individual organisations a prefix within the identifier system.

One challenge with prefix allocation is ensuring that the rules for locally assigned suffixes work in every context where the identifier needs to appear.

In distributed assignment of identifiers, anyone can create an identifier. Rather than requesting an identifier, or a prefix from a registration agency, these systems operate by agreeing rules for how unique identifiers can be constructed.

A hash based identifier takes some properties of the thing we want to identify and then use that to construct an identifier.