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Internet Protocol version 4 (IPv4) is one of the core protocols of the Internet Protocol Suite, and it is defined in RFC 791. Published in September 1981, IPv4 was the first version of the Internet Protocol to be widely deployed and remains one of the fundamental technologies underpinning the modern internet. IPv4 provides the means for addressing and routing packets of data from a source to a destination across multiple networks.

A key feature of IPv4 is its use of 32-bit addresses, which limits the address space to approximately 4.3 billion unique IP addresses. While this seemed sufficient during the early days of the internet, the explosive growth of internet-connected devices has led to a depletion of available IPv4 addresses. This limitation has been one of the driving factors behind the development and deployment of IPv6, which offers a vastly expanded address space.

IPv4 operates using a connectionless model, where data is broken into smaller packets and transmitted independently through the network. Each packet contains a header that includes the source and destination IP addresses, along with other information necessary for routing and delivery. Unlike some other protocols, IPv4 does not guarantee reliable delivery of packets, leaving error detection and retransmission to higher-layer protocols like TCP.

The IPv4 header contains various fields that are critical to the protocol's operation. These include the version field, which indicates that the packet is using IPv4, the header length, type of service, total length, identification, flags, fragment offset, time-to-live (TTL), and protocol. These fields allow routers to efficiently process and forward packets while ensuring that they do not remain in the network indefinitely.

Another important feature of IPv4 is its support for fragmentation and reassembly. If a packet is too large to be transmitted across a particular network, IPv4 allows it to be fragmented into smaller packets, which are reassembled by the recipient. This ensures that IPv4 can operate over a wide variety of network types with differing maximum transmission unit (MTU) sizes.

Despite its long-standing use, IPv4 has several limitations. As mentioned, the most pressing issue is the exhaustion of available addresses, which has led to the widespread use of techniques like Network Address Translation (NAT) to extend the lifespan of IPv4. However, NAT introduces additional complexity and can sometimes interfere with the end-to-end connectivity of applications.

For further technical details, refer to these sources: - RFC 791: https://www.rfc-editor.org/info/rfc791 - Wikipedia on IPv4: https://en.wikipedia.org/wiki/IPv4

IPv4 remains a critical component of the internet's infrastructure, facilitating the communication and routing of data across global networks. Although it has been superseded in many ways by IPv6, IPv4 continues to be widely used due to its ubiquity and the vast infrastructure that has been built around it. Understanding IPv4 is essential for network engineers and IT professionals, as it remains relevant in many networks, even as the transition to IPv6 progresses.

Snippet from Wikipedia: IPv4

Internet Protocol version 4 (IPv4) is the first version of the Internet Protocol (IP) as a standalone specification. It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version deployed for production on SATNET in 1982 and on the ARPANET in January 1983. It is still used to route most Internet traffic today, even with the ongoing deployment of Internet Protocol version 6 (IPv6), its successor.

IPv4 uses a 32-bit address space which provides 4,294,967,296 (2³²) unique addresses, but large blocks are reserved for special networking purposes.

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