Networking / Beginners

Security Identifiers (SIDs)

Every user in Windows NT is represented by a unique security identifier (SID). SIDs for users always start with S-1-5-21 in Windows, denoting that they are issued by the NT identifying authority and that they may not be unique within the universe of SIDs. User SIDs are based on machine SIDs. For instance, S-1-5-21-1095672315-1787444531-3518664281 may be the SID of a machine. Users, in turns, will have SIDs based on the machine or domain SID, with a relative identifier (RID) appended. For example, the administrator on the machine will have a SID of S-1-5-21-1095672315-1787444531-3518664281-500.

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