CAT6 wire pair details?

What each wire in the cable does depends on how the cable is wired to the Ethernet connector. In addition, to allow straight-through cables to work, end nodes typically have transmit/receive reversed with respect to switches. Most modern devices detect this automatically anyway.

For 100-base-T, the standard for wired fast Ethernet connectors, the connector wirind is:
1 - TX+
2 - TX-
3 - RX+
6 - RX-
The remaining 4 pins are unused. Note that 1/2 form a pair and should be wired to a twisted pair in the cable. Similarly, 3/6 form a pair and should be wired to a twisted pair in the cable. A typical CAT5 or CAT6 four pair run can carry two fast Ethernet links since each only needs two pairs.

For Gigabit, it's:
1 - A+
2 - A-
3 - B+
4 - C+
5 - C-
6 - B-
7 - D+
8 - D-
The 8 pins form four bi-directional pairs. Each pair handles differential signals in both directions at all times. 1/2 form a pair, 3/6 form a pair, 4/5 form a pair, and 7/8 form a pair. Pairs must be mapped onto twisted pairs in the cable.

It should be noted that POE comes into play here as well. 10BASE-T and 100BASE-TX Ethernet use only two pairs of wire in 4-pair CAT5/CAT5e/CAT6 cable, leaving the other two pairs free to transmit power for Power over Ethernet (PoE) applications.

However, Gigabit Ethernet or 1000BASE-T uses all four pairs of wires, leaving no pairs dedicated for power.

Through the use of phantom power—power sent over the same wire pairs used for data, the same pair is used for both power and data, the power and data transmissions don’t interfere with each other because electricity and data function at opposite ends of the frequency spectrum, they can travel over the same cable. Electricity has a low frequency of 60 Hz or less, and data transmissions have frequencies that can range from 10 million to 100 million Hz.

10- and 100-Mbps PoE may also use phantom power. The 802.3af PoE standard for use with 10BASE-T and 100BASE-TX defines two methods of power transmission. In one method, called Alternative A, power and data are sent over the same pair. In the other method, called Alternative B, two wire pairs are used to transmit data, and the remaining two pairs are used for power. PoE Powered Devices (PDs) are made to accept power in either format.