Besides an IP address, online identifiers include cookie IDs, RFID tags, MAC addresses, mobile advertising IDs, pixel tags, account handles and device fingerprints. Names, identification numbers and location data can also help identify someone; phone numbers and email addresses are common contact identifiers. What an identifier points to—and whether it reveals a person’s real-world identity—depends on the system and the information that can be linked to it.
Common types of identifiers besides an IP address
An identifier is not necessarily a person’s name. Depending on where it is used, it may distinguish a device, account, organization, session or individual. These examples are useful categories, not a complete list.
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| Identifier type | Examples | What it may distinguish |
|---|---|---|
| Browser and web identifiers | Cookie IDs, pixel tags | A browser or online activity in a particular system; traces may be combined with other information to build a profile. |
| Device and hardware identifiers | MAC addresses, RFID tags, device fingerprints | A device or, in some contexts, a user associated with it. A device identifier does not automatically identify the person using the device. |
| Advertising identifiers | Google Advertising ID, Apple ID for Advertisers | A mobile advertising identifier used within relevant systems. These examples appear in a 2025 U.S. Department of Justice rule. |
| Account identifiers | Usernames, account handles | An account or online user. A handle can distinguish someone without disclosing their legal name. |
| Contact and conventional identifiers | Name, phone number, email address, identification number | A person or contact route, depending on the value and context. A name alone may be ambiguous. |
| Contextual or derived identifiers | Location data, combinations of attributes | Potentially a person or other entity when enough surrounding information is available. |
Does a unique identifier reveal who someone is?
No—not by itself in every case. “Unique” usually means that a value distinguishes something within a particular system or context. It does not guarantee that the value reveals a person’s civil or real-world name.
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For example, a username may uniquely distinguish an account while leaving the user’s legal identity unknown. Conversely, an otherwise ordinary detail can become identifying when combined with other information. The UK Information Commissioner’s Office (ICO) explains that identifiability depends on context and that identifiers may work alone or in combination. It gives the example: “By itself, the name ‘John Smith’ may not always be personal data because there are many individuals with that name.” ICO guidance on identifiers and related factors says its list of identifiers is non-exhaustive and carries a notice that the guidance is under review.
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Why combinations matter
One identifier can be weak evidence of identity; several linked together may reveal much more. A cookie ID, location data, account information or device identifier may become more informative when associated with other records. The key question is not only what a value is called, but what entity it distinguishes and what other data can be connected to it.
The National Institute of Standards and Technology (NIST) uses “identifier” broadly: it can refer to data representing a person’s identity and attributes, a bit string associated with a person, device or organization, or—depending on context—a data object associated with one unique entity such as an individual, device or session. NIST’s glossary definition helps explain why the same general term can refer to different kinds of things.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to read identifiers in a legal context
Legal categories depend on the law and the purpose of the document; there is no single list that applies everywhere. A January 8, 2025 U.S. Department of Justice rule names device and hardware identifiers such as IMEI, MAC and SIM numbers; demographic and contact data; advertising identifiers; account-authentication data; network identifiers such as IP addresses and cookie data; and call-detail data. It also discusses how combinations can make identifiers linkable to individuals, including the possibility that an IP address contributes to identification even when it is shared. This is a description within that specific U.S. federal rule, not a universal taxonomy or a summary of privacy law in every jurisdiction. Read the Federal Register rule.
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A practical way to distinguish the terms
- Ask what is being identified: a person, device, account, organization or session.
- Ask where the identifier works: a value may distinguish something in one service or system without identifying it everywhere.
- Ask what can be linked to it: other records or attributes may change how identifying it is.
- Separate identification from naming: distinguishing an account or device does not necessarily disclose the user’s real-world name.
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