Hairpin references telephone systems and the process of sending a call back in the direction of its origination. If a call cannot be directed over Internet Protocol to a gateway closer to the target telephone, the call is often redirected back to the local zone, which is the direction of its origination.
The term hairpin comes from the shape of women's hairpin, with its two legs. In telephony, a single call may be turned into a two-legged call. The two legs intersect at the destination number. The first leg is the call from the source number to the destination number, while the second is the call redirected back from the called number. Additionally, the term hairpin was later used in digital data networks to denote an existing feature in some network address translations (NATs) where more than one host is placed behind a NAT. Each host is assigned a certain external address and port by the NAT.
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