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IPv4, or Internet Protocol version 4, is a network-layer protocol that moves datagrams from source to destination across interconnected networks. It uses 32-bit addresses, but it does not by itself guarantee that data arrives, arrives in order, or is retransmitted if lost.
What is IPv4?
IPv4 is the fourth version of the Internet Protocol. Its purpose is to carry datagrams across interconnected networks; routers forward them using the destination address. As Jon Postel put it in RFC 791, published in September 1981, “The function or purpose of Internet Protocol is to move datagrams through an interconnected set of networks.”
IPv4 is a protocol, while an IPv4 address is an address used by that protocol. Each datagram is handled independently: IPv4 does not establish a virtual circuit or provide end-to-end reliability, sequencing, or flow control. Those functions, when needed, are supplied by protocols or mechanisms outside IPv4.
What does an IPv4 address look like?
An IPv4 address contains 32 bits, divided into four 8-bit units called octets. It is commonly written as four decimal numbers separated by periods, with each number representing one octet—for example, 192.0.2.1. The 32-bit size is specified in RFC 791.
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What information is in an IPv4 datagram header?
The header carries information needed to identify and forward a datagram, along with fields that support its handling. RFC 791 specifies a version field, header length, service indication, total length, identification, fragmentation flags and offset, Time to Live (TTL), protocol identifier, header checksum, source and destination addresses, and optional fields.
- Version: A four-bit field identifying the header format; IPv4 uses the value 4.
- Source and destination addresses: Each address occupies four octets.
- Total Length: A 16-bit field giving the length of the entire datagram, including its header and data. The specification permits a maximum of 65,535 octets; that field limit does not mean a datagram of that size can travel across every network in practice.
- Header length: The Internet Header Length field counts 32-bit words. Its specified range corresponds to a header from 20 octets (five words) to 60 octets (15 words).
- Fragmentation fields: Identification, flags, and offset support fragmentation and reassembly when a datagram must be handled across networks with different packet-size limits.
Does IPv4 guarantee delivery?
No. IPv4 provides addressing and datagram-forwarding mechanisms, but an IPv4 datagram can be lost, duplicated, or delivered out of order. IPv4 itself does not promise delivery, retransmission, sequencing, or flow control. A higher-layer protocol or application may provide some of these functions, but they should not be attributed to IPv4 alone.
How does IPv4 handle different network packet-size limits?
Networks can differ in the largest packet they can carry. IPv4 specifies fragmentation and reassembly mechanisms to handle datagrams that exceed a network’s packet-size limit. The header’s identification, flags, and fragment offset fields help describe fragments and their relationship to the original datagram. The protocol specification’s maximum total length of 65,535 octets is a field limit, not a practical guarantee that every network can carry a datagram of that size.
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The version field in the IPv4 header identifies the format as version 4. It is a protocol version identifier, not a statement about how many devices or networks are involved. RFC 791 is the foundational specification, published in September 1981; the RFC Editor notes that parts of the document have since been updated, so its original field descriptions should not be treated as proof that every detail remains unchanged in current deployments.
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