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
The size of a block of addresses is specified by writing a slash (/) followed by a number in decimal whose value is the length of the network prefix in bits, rather than by explicitly specifying which addresses are in the block. For example, an address block with 48 bits in the prefix is indicated by . Such a block contains 2 = 2 addresses. The smaller the value of the network prefix, the larger the block: a block is 8 times larger than a block.