The header of an IP packet is an essential data structure, divided into several fields, each with a specific role to ensure the packets are transmitted and processed correctly as they travel through the network. These fields include the destination IP address (needed to route the packet to its intended recipient), the header length indicated by the IHL (Internet Header Length) field, the total packet length recorded in the Total Length field, control and verification information, and other parameters for managing communication flow and quality.

The very first field in the header is called Version. This 4-bit value specifies which version of the IP protocol the packet follows. It's important because it tells each router or intermediate device how to interpret and handle the encapsulated data.

Note: The management and assignment of IP protocol versions is under the responsibility of IANA. A 4-bit field allows for 16 binary combinations (values 0 to 15). As of today, their assignment is as follows:

Among these is IPv5, which, although largely unknown to the public, did exist as the ST (Stream Protocol). Developed in the 1980s, IPv5 was designed to address a growing need at the time: providing "Quality of Service" (QoS) for certain data flows that required continuous, stable transmission, such as Voice over IP or multimedia streams. Its goal was to guarantee end-to-end bandwidth and priority, a concept similar to what the RSVP (Resource Reservation Protocol) offers today for dynamically reserving network resources on modern routers.

However, IPv5 remained experimental and was implemented on only a small number of network devices. Its limited adoption, combined with the rapidly growing need for more address space, led Internet designers to skip directly from IPv4 to IPv6. This avoided both IPv4's address limitations and any risk of confusion or incompatibility with IPv5's experimental specifications.

Although IPv5 never saw widespread use, it played an important role in shaping early thinking about QoS and traffic management. Today, it is more of a historical marker than a working standard.

Reminder - A protocol is a set of communication rules: data structures, algorithms, packet formats, and conventions that allow different devices to exchange information reliably and understandably. A service is the concrete implementation of a protocol through specific programs (clients, servers) that follow these rules and make the functionality available to users and applications.

We can now take a closer look at the structure and operation of the IP protocol, the essential foundation of all network communication.