IP address
An IP address serves two principal functions. It identifies the host, or more specifically its network interface, and it provides the location of the host in the network, and thus the capability of establishing a path to that host. Its role has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."
The header of each IP packet contains the IP address of the sending host, and that of the destination host.
IPv6 address
A 64-bit interface identifier is most commonly derived from its 48-bit MAC address. A MAC address is turned into a 64-bit EUI-64 by inserting in the middle: . When this EUI-64 is used to form an IPv6 address it is modified: the meaning of the "Universal/Local" bit (the 7th most significant bit of the EUI-64, starting from 1) is inverted, so that a 1 now means "Universal". To create an IPv6 address with the network prefix it yields the address (with the "Universal/Local" bit, the second-least-significant bit of the underlined quartet, inverted to 1 in this case because the MAC address is universally unique).