The best way to understand Internet Protocol version 4 (IPv4) is to know the IPv4 header and all its fields. Segments from Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) are passed on to IP for processing. The IP header is appended to the TCP or UDP segment. The TCP or UDP segment then becomes the IP data. The IPv4 header is 20 bytes in length when it uses no optional fields. The IP header includes the addresses of the sending host and the destination host. It also includes the upper-layer protocol, a field for prioritization, and a field for fragmentation. Figure 1-1 shows the IP header format.