SI unit prefix meaning 1,000 (or one thousand). In networking terms, the kilo prefix is normally associated with the link capacity (bandwidth) of a low speed network link, such as RS-232 serial link, or a domestic telephone modem. In this context the term baud is often used to define the “line speed”, where 9,600 baud is 9.6 kbit/s.

A common error made by computing students is to assume that, because powers of two are used everywhere else, it is used in the definition of bandwidth too. That is 1 kbit/s should be 2^10 (= 1,024) bit/s: it isn't! The SI system uses the prefix ki (pronounced kibi) to represent 2^10: thus when computer manufacturers quote file-sizes in terms of kilobytes (when they do intend to mean that 1 kbyte is 1024 bytes) they should actually use 1 kibyte.

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