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IPv4 is version 4 of the Internet Protocol (IP). It was the first version of the Internet Protocol to be widely deployed, and forms the basis for most of the current Internet ( as of 2004).

It is described in IETF RFC 791, which was first published in September, 1981.

IPv4 uses 32-bit addresses, limiting it to 4,294,967,296 unique addresses, many of which are reserved for special purposes such as local networks or multicast addresses, reducing the number of addresses that can be allocated as public Internet addresses.

As the number of addresses available is consumed, an IPv4 address shortage appears to be inevitable in the long run.

This limitation has helped stimulate the push towards IPv6, which is currently in the early stages of deployment, and is hoped will eventually replace IPv4.

1 IPv4 address representations

IPv4 addresses are usually written in Dot-decimal notation. Here's an example: 207.142.131.235. But it's also possible to write in the following formats:

The above ip addresses should work in most browsers and at the time of writing point to wikipedia.org.

2 IPv4 header format

The first header field in an IPv4 packet is the 4-bit version field.

The second field is a 4-bit Internet Header Length (IHL) telling the number of 32-bit words in the IPv4 header. Since an IPv4 header may contain a variable number of options, this field essentially specifies the offset to the data portion of an IPv4 datagram. A minimum IPv4 header is 20 bytes long, so the minimum value in decimal in the IHL field would be 5.

In RFC 791, the following 8 bits were allocated to a Type of Service (ToS) field - now DiffServ and ECN. The original intention was for a sending host to specify a preference for how the datagram would be handled as it made its way through an internetwork. For instance, one host could set its IPv4 datagrams' ToS field value to prefer low delay, while another might prefer high reliability. In practice, the ToS field has not been widely implemented. However, a great deal of experimental, research and deployment work has focused on how to make use of these eight bits. These bits have been redefined and most recently through DiffServ working group in the IETF and the Explicit Congestion Notification codepoints (see RFC 3168).

The next 16-bit IPv4 field defines the entire datagram size, including header and data, in 8-bit bytes. The minimum-length datagram is 20 bytes and the maximum is 65535. The maximum size datagram which any host is required to be able to handle is 576 bytes, but most modern hosts handle much larger packets. Sometimes subnetworkThe word subnetwork has two related meanings. In the older and more general meaning, it meant one physical network of an internet. In the Internet Protocol, a subnetwork, usually known as a subnet is a division of a computer network. The rest of this artis impose further restrictions on the size, in which case datagrams must be fragmented. Fragmentation is handled in either the host or packet switch in IPv4 (see below).

The next 16-bit field is an identification field. This field is primarily used for uniquely identifying fragments of an original IP datagram. Some experimental work has suggested using the ID field for other purposes, such as for adding packet tracing information to datagrams in order to help trace back datagrams with spoofed source addresses.

A 3-bit field follows and is used to control or identify fragments. They are (in order, from high order to low order:

• Reserved, must be zero
• Don't Fragment (if set)
• More Fragments.

The fragment offset field is 13-bits long, and allows a receiver to determine the place of a particular fragment in the original IP datagram, measured in units of 8-byte blocks.

An 8-bit time to liveTime to live TTL is an 8-bit field in the Internet Protocol (IP) header that indicates how many more hops this packet should be allowed to make before being discarded or returned. It is the 9th octet of 20 in the IP header. TTL's also occur in the Domain (TTL) field helps prevent datagrams from persisting (e.g. going in circles) on an internetwork. Historically the TTL field limited a datagram's lifetime in seconds, but has come to be a hop countIn telecommunication, the term hop count has the following meanings: #In a data communications network, the number of legs traversed by a packet between its source and destination. Note Hop count may be used to determine the time to live for some packets. field. Each packet switch (or routerThis article describes the computer networking device. A wood router is also a kind of rotating cutting tool. NAT Router, popular for home and small office networks A router is a computer networking device that forwards data packets toward their destinati) that a datagram crosses decrements the TTL field by one. When the TTL field hits zero, the packet is no longer forwarded by a packet switch and is discarded.

An 8-bit Protocol field follows. This field defines the next protocol used in the data portion of the IP datagram. The Internet Assigned Numbers AuthorityThe Internet Assigned Numbers Authority (IANA) is an organisation that oversees IP address, top level domain and Internet protocol code point allocations. IANA was formerly one man, the late Jon Postel General information IANA delegates local registration maintains a list of Protocol numbers. Common protocols and their decimal values include ICMP (1), TCPTransmission Control Protocol (TCP is a connection-oriented, reliable delivery byte-stream transport layer communication protocol, currently documented in IETF RFC 793. It does the task of the transport layer in the simplified OSI model of computer networ (6) and UDP (17).

The following field is a 16-bit checksum field for the IPv4 datagram header. Some values in a IPv4 datagram header may change at each packet switch hop, so the checksum must be adjusted on its way through an internetwork.

The checksum is followed by a 32-bit source address and 32-bit destination address respectively.

Additional header fields (called options) may follow the destination address field, but these are not often used. Option fields may be followed by a pad field which ensures that the user data are aligned on a 32-bit word boundary.

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IPv4IPv4 is version 4 of the Internet Protocol (IP). It was the first version of the Internet Protocol to be widely deployed, and forms the basis for most of the current Internet ( as of 2004). It is described in IETF RFC 791, which was first published in Sep IPv4 subnetting referenceCertain chunks of IPv4 address space are specially allocated by RFCs for special uses such as loopback, RFC 1918 networks, and Zeroconf usage, and are not available for allocation by Regional Internet Registries (RIRs). The netmask is a bitmask that can b IPv4 address exhaustionAs the number of Internet-connected devices increases, the address space of the current IPv4 version of the TCP/IP protocol suite is rapidly being consumed by allocations for new devices. This creates an IPv4 address shortage and could conceivably result