An IPv4 address is a 32-bit address that uniquely and universally defines the connection of a device (for example, a computer or a router) to the Internet. IPv4 addressing, at its inception, used the concept of classes. This architecture is called classful addressing. In classful addressing, the address space is divided into five classes: A, B, C, D, and E. Each class occupies some part of the address space. IP address is denoted by binary notation or dotted-decimal notation.

An IPv4 datagram consists of a header part and a body or payload part. The header has a 20-byte fixed part and a variable-length optional part.

A brief description of each field is in order.

• Version (VER): This 4-bit field defines the version of the IPv4 protocol. Currently the version is 4. However, version 6 (or IPV6) may totally replace version 4 in the future. This field tells the IPv4 software running in the processing machine that the datagram has the format of version 4. All fields must be interpreted as specified in the fourth version of the protocol. If the machine is using some other version ofIPv4, the datagram is discarded rather than interpreted incorrectly.

• Header length (HLEN): This 4-bit field defines the total length of the datagram header in 4-byte words. This field is needed because the length of the header is variable (between 20 and 60 bytes). When there are no options, the header length is 20 bytes, and the value of this field is 5 (5 x 4 = 20). When the option field is at its maximum size, the value of this field is 15 (15 x 4 = 60).

• Services: IETF has changed the interpretation and name of this 8-bit field. This field, previously called service type, is now called differentiated services. We show both interpretations in Figure below.

Service Type

In this interpretation, the first 3 bits are called precedence bits. The next 4 bits are called type of service (TOS) bits and the last bit is not used.

a. Precedence is a 3-bit subfield ranging from 0 (000 in binary) to 7 (111 in binary). The precedence defines the priority of the datagram in issues such as congestion. If a router is congested and needs to discard some datagrams, those datagrams with lowest precedence are discarded first. Some datagrams in the Internet are more important than others.

b. TOS bits are a 4-bit subfield with each bit having a special meaning. Although a bit can be either 0 or 1, one and only one of the bits can have the value of 1 in each datagram

The Differentiated services field is one of the few fields that has changed its meaning (slightly) over the years. Originally, it was called the Type of service field. It was and still is intended to distinguish between different classes of service.

Total length:

• This is a 16-bit field that defines the total length (header plus data) of the IPv4 datagram in bytes. To find the length of the data coming from the upper layer, subtract the header length from the total length. The header length can be found by multiplying the value in the HLEN field by 4.
• Length of data =total length - header length Since the field length is 16 bits, the total length of the IPv4 datagram is limited to65,535 (216 - 1) bytes, of which 20 to 60 bytes are the header and the rest is data from the upper layer.

Identification, Flags, Fragmentation offset:

This three fields in the IP header have been assigned to manage fragmentation and reassembly. At the destination IP has to collect fragments for reassembling into packets.

• The identification field is used to identify which packet a particular fragment belongs to so that fragments for different packets do not get mixed up. To have a safe operation, the source host must not repeat the identification value of the packet destined to the same host until a sufficiently long period of time has passed.

• The flags field has three bits: one unused bit, one “don’t fragment”(DF) bit, and one “more fragment”(MF) bit. If the DF bit is set to 1, it forces the router not to fragment the packet. If the packet length is greater than the MTU, the router will have to discard the packet and send an error message to the source host. If there are more, the MF bit is set to 1; otherwise it is set to 0. The fragment offset field identifies the location of a fragment in a packet. The value measures the offset, in units of eight bytes between the beginning of the packet to be fragmented and the beginning of the fragment, considering the data part only. Thus the first fragment of a packet has an offset value of 0.

Time to live: A datagram has a limited lifetime in its travel through an internet. This field was originally designed to hold a timestamp, which was decremented by each visited router. The datagram was discarded when the value became zero. However, for this scheme, all the machines must have synchronized clocks and must know how long it takes for a datagram to go from one machine to another. Today, this field is used mostly to control the maximum number of hops (routers) visited by the datagram. When a source host sends the datagram, it stores a number in this field. This value is approximately 2 times the maximum number of routes between any two hosts. Each router that processes the datagram decrements this number by 1.

• If this value, after being decremented, is zero, the router discards the datagram. This field is needed because routing tables in the Internet can become corrupted.
• A datagram may travel between two or more routers for a long time without ever getting delivered to the destination host. This field limits the lifetime of a datagram.
• Another use of this field is to intentionally limit the journey of the packet. For example, if the source wants to confine the packet to the local network, it can store 1 in this field. When the packet arrives at the first router, this value is decremented to 0, and the datagram is discarded.

Protocol: This 8-bit field defines the higher-level protocol that uses the services of the IPv4 layer. An IPv4 datagram can encapsulate data from several higher-level protocols such as TCP, UDP, ICMP, and IGMP. This field specifies the final destination protocol to which the IPv4 datagram is delivered. In other words, since the IPv4 protocol carries data from different other protocols, the value of this field helps the receiving network layer know to which protocol the data belong

Checksum: The implementation of the checksum in the IPv4 packet follows the same principles. First, the value of the checksum field is set to 0. Then the entire header is divided into 16-bit sections and added together. The result (sum) is complemented and inserted into the checksum field. The checksum in the IPv4 packet covers only the header, not the data.

Source & Destination address: This 32-bit field defines the IPv4 address of the source and destination respectively. This field must remain unchanged during the time the IPv4 datagram travels from the source host to the destination host.

Options: The header of the IPv4 datagram is made of two parts: a fixed part and a variable part.

The fixed part is 20 bytes long and the variable part comprises the options that can be a maximum of 40 bytes. Options, as the name implies, are not required for a datagram. They can be used for network testing and debugging. Although options are not a required part of the IPv4 header, option processing is required of the IPv4 software. This means that all implementations must be able to handle options if they are present in the header.