The data fields of the IP header
The first data field of the IP header is the version field, a 4-bit long field to identify the protocol version. The following header length( IHL) data field is also four bits long and indicates the total length of the IP header. The length is specified in 32-bit units. The specification is required because the option field has a variable length. The service field following the length field, Type of Service( ToS), is an 8-bit long data field that specifies the service with its network-relevant transmission parameters. Various parameters for the priority handling of datagrams, the throughput type and the allocation of resources in the routers are specified in this data field. The following data field Total Length specifies the datagram length. It is 16 bits long and thus represents a maximum datagram length of 64 Kbytes.
The identifier field, which is also 16 bits long, is used to uniquely identify individual data packets. The flag field following the identifier field, which consists of 3 flag bits, indicates whether the datagrams are fragmented or not and whether they have to be reassembled in the receiving data station. The following 13-bit fragment offset field indicates the location of the fragment data with respect to the beginning of the datagram.
To prevent datagrams with incorrect information from wandering around the network unnecessarily and thus generating unnecessary data traffic, they are provided with a lifetime information. The 8 bit long time-to-live field( TTL) is used to specify the lifetime of the datagram.
The protocol type field is an 8-bit long field in the IP header that is used to demultiplex incoming datagrams and determines the protocol type of the next higher layer. In the Header Checksum field, which is 16 bits long, only the IP header ischecked for errors. In the Source Address Field, which is 32 bits long, the IP address of the network node that sent the IP datagram is entered. The address of the recipient of the datagram, the Destination Address Field, is also 32 bits long. The encoding of the IP addresses is hexadecimal. Up to the Destination Address Field the total length of the IP header is 20 bytes.
The option field, if present, is used to adapt services to the higher protocols. For example, this field can be used to specify certain security requirements that the receiving side must meet. The option field has a variable length of up to 40 bytes and is therefore always padded to a multiple of 32 bits by the padding.