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in 100BaseTX applications there are only used 4 pins (symm TxD, symm RxD) the other ones are just connected one to one in the connector and are used in so called structured wiring. (e.g. if you are cabling a company you just need one type of cable and use i for ethernet, telephone, isdn, and other communication devices)
In Cat.6 ot higher all pins are used due to the higher bandwith but with the exact same pinning. this is extremely useful when changing the IT you are able to use the same wiring.