Often all 8 wires are in use with Power over Ethernet. But sometimes 2-4 wires can go unused on a 10/100 link.
Via wikipedia:
[TABLE="class: wikitable"]
802.3af Standards A and B from the power sourcing equipment perspective[TR]
[TH="bgcolor: #F2F2F2, align: center"]Pins at switch[/TH]
[TH="bgcolor: #F2F2F2, align: center"]T568A color[/TH]
[TH="bgcolor: #F2F2F2, align: center"]T568B color[/TH]
[TH="bgcolor: #F2F2F2, colspan: 2, align: center"]10/100 mode B,
DC on spares[/TH]
[TH="bgcolor: #F2F2F2, colspan: 2, align: center"]10/100 mode A,
mixed DC & data[/TH]
[TH="bgcolor: #F2F2F2, colspan: 2, align: center"]1000 (1 gigabit) mode B,
DC & bi-data[/TH]
[TH="bgcolor: #F2F2F2, colspan: 2, align: center"]1000 (1 gigabit) mode A,
DC & bi-data[/TH]
[/TR]
[TR]
[TH="bgcolor: #F2F2F2, align: center"]Pin 1[/TH]
[TD]

White/green stripe[/TD]
[TD]

White/orange stripe[/TD]
[TD]Rx +[/TD]
[TD][/TD]
[TD]Rx +[/TD]
[TD]DC +[/TD]
[TD]TxRx A +[/TD]
[TD][/TD]
[TD]TxRx A +[/TD]
[TD]DC +[/TD]
[/TR]
[TR]
[TH="bgcolor: #F2F2F2, align: center"]Pin 2[/TH]
[TD]

Green solid[/TD]
[TD]

Orange solid[/TD]
[TD]Rx −[/TD]
[TD][/TD]
[TD]Rx −[/TD]
[TD]DC +[/TD]
[TD]TxRx A −[/TD]
[TD][/TD]
[TD]TxRx A −[/TD]
[TD]DC +[/TD]
[/TR]
[TR]
[TH="bgcolor: #F2F2F2, align: center"]Pin 3[/TH]
[TD]

White/orange stripe[/TD]
[TD]

White/green stripe[/TD]
[TD]Tx +[/TD]
[TD][/TD]
[TD]Tx +[/TD]
[TD]DC −[/TD]
[TD]TxRx B +[/TD]
[TD][/TD]
[TD]TxRx B +[/TD]
[TD]DC −[/TD]
[/TR]
[TR]
[TH="bgcolor: #F2F2F2, align: center"]Pin 4[/TH]
[TD]

Blue solid[/TD]
[TD]

Blue solid[/TD]
[TD][/TD]
[TD]DC +[/TD]
[TD="class: table-na, colspan: 2, align: center"]Unused[/TD]
[TD]TxRx C +[/TD]
[TD]DC +[/TD]
[TD]TxRx C +[/TD]
[TD][/TD]
[/TR]
[TR]
[TH="bgcolor: #F2F2F2, align: center"]Pin 5[/TH]
[TD]

White/blue stripe[/TD]
[TD]

White/blue stripe[/TD]
[TD][/TD]
[TD]DC +[/TD]
[TD="class: table-na, colspan: 2, align: center"]Unused[/TD]
[TD]TxRx C −[/TD]
[TD]DC +[/TD]
[TD]TxRx C −[/TD]
[TD][/TD]
[/TR]
[TR]
[TH="bgcolor: #F2F2F2, align: center"]Pin 6[/TH]
[TD]

Orange solid[/TD]
[TD]

Green solid[/TD]
[TD]Tx −[/TD]
[TD][/TD]
[TD]Tx −[/TD]
[TD]DC −[/TD]
[TD]TxRx B −[/TD]
[TD][/TD]
[TD]TxRx B −[/TD]
[TD]DC −[/TD]
[/TR]
[TR]
[TH="bgcolor: #F2F2F2, align: center"]Pin 7[/TH]
[TD]

White/brown stripe[/TD]
[TD]

White/brown stripe[/TD]
[TD][/TD]
[TD]DC −[/TD]
[TD="class: table-na, colspan: 2, align: center"]Unused[/TD]
[TD]TxRx D +[/TD]
[TD]DC −[/TD]
[TD]TxRx D +[/TD]
[TD][/TD]
[/TR]
[TR]
[TH="bgcolor: #F2F2F2, align: center"]Pin 8[/TH]
[TD]

Brown solid[/TD]
[TD]

Brown solid[/TD]
[TD][/TD]
[TD]DC −[/TD]
[TD="class: table-na, colspan: 2, align: center"]Unused[/TD]
[TD]TxRx D −[/TD]
[TD]DC −[/TD]
[TD]TxRx D −[/TD]
[TD][/TD]
[/TR]
[/TABLE]