Whatever application protocol is used to implement an Internet service, IP lies at the heart of all Internet data communications. IP is a datagram protocol, which means transmitted packets of information (packets, for short) are not guaranteed to be delivered. IP packets also do not form part of a stream of related packets; IP is a connectionless protocol. Each IP packet travels on its own, like an individual letter in a postal network (or a guru looking for enlightenment).
Instead of writing down 32-digit long bit-strings, like 11001110110000110001011111010000, Internet addresses are almost always expressed in their human-readable, textual form (for example, www.google.com). On the rarer occasions when the address need to be expressed numerically, these 32-bit IP addresses are written as four decimal bytes (for example, 192.31.32.225). The remainder of the header encodes a collection of fields, including the total packet length in bytes. Sixteen bits are allocated for this field, so an IP packet can be a maximum of 64KB long.