opular and widely used of the internetworking protocols. The nonproprietary nature of the suite is a major advantage, permitting the con- nection of hardware and operating systems of many different computers. Internet Basics IP handles packet forwarding and transporting of datagrams across a network. With packet forwarding, computers can send a packet on to the next appropriate network component, based on the address in the packet's header. IP defines the basic unit of data transfer, the datagram, also referred to as the packet, and it also defines the exact format of all data as it travels across the Internet. IP works like an envelope in the postal service, directing information to its proper destination. With this arrangement, every computer on the Internet has a unique address. (Address- ing is discussed later in this chapter.) Figure 8.5 shows the various components of a datagram. The entire datagram has four entities-the Payload field, the next protocol field (usually TCP or UDP), the IP field, and the L2 (Layer 2) field-which, combined, are referred to as the Layer 2 packet (or protocol) data unit (PDU), or frame. Datagram, or packet, refers to the combination of the payload, the TCP/UDP information, and the IP address. Finally, the TCP maximum segment size (MSS) refers to the size of the payload and the next protocol header. IP provides software routines to route and to store and forward data among hosts on the network. IP functions at Layer 3 (the network layer), and it provides several services, including host addressing, error notification, fragmentation and reassembly, routing, and packet timeout. The Layer 4 protocol, usually TCP or UDP, presents the data to IP in order to provide basic host-to-host communication. IP then attaches to the packet, in a protocol header, the address from which the data comes and the address of the system to which it is going. Under the standards, IP allows a packet size of up to 64,000 bytes, but we don't transmit packets that large because they would cause session timeo