Note:
taken from http://www.microsoft.com/windows2000/docs/ipv6.doc
IPv4 Header
Figure 18 shows the IPv4 header described in RFC 791.
Figure 18 The IPv4 header
The fields in
the IPv4 header are:
Version Indicates the version of IP and is set to 4. The size of this field is 4 bits.
Internet Header Length Indicates the number of 4-byte blocks in the IPv4 header. The size of this field is 4 bits. Because an IPv4 header is a minimum of 20 bytes in size, the smallest value of the Internet Header Length (IHL) field is 5. IPv4 options can extend the minimum IPv4 header size in increments of 4 bytes. If an IPv4 option does not use all 4 bytes of the IPv4 option field, the remaining bytes are padded with 0s, making the entire IPv4 header an integral number of 32-bits (4 bytes). With a maximum value of 0xF, the maximum size of the IPv4 header including options is 60 bytes (15΄4).
Type of Service Indicates the desired service expected by this packet for delivery through routers across the IPv4 internetwork. The size of this field is 8 bits, which contain bits for precedence, delay, throughput, and reliability characteristics.
Total Length Indicates the total length of the IPv4 packet (IPv4 header + IPv4 payload) and does not include link layer framing. The size of this field is 16 bits, which can indicate an IPv4 packet that is up to 65,535 bytes long.
Identification Identifies this specific IPv4 packet. The size of this field is 16 bits. The Identification field is selected by the originating source of the IPv4 packet. If the IPv4 packet is fragmented, all of the fragments retain the Identification field value so that the destination node can group the fragments for reassembly.
Flags Identifies flags for the fragmentation process. The size of this field is 3 bits, however, only 2 bits are defined for current use. There are two flagsone to indicate whether the IPv4 packet might be fragmented and another to indicate whether more fragments follow the current fragment.
Fragment Offset Indicates the position of the fragment relative to the original IPv4 payload. The size of this field is 13 bits.
Time to Live Indicate the maximum number of links on which an IPv4 packet can travel before being discarded. The size of this field is 8 bits. The Time-to-Live field (TTL) was originally used as a time count with which an IPv4 router determined the length of time required (in seconds) to forward the IPv4 packet, decrementing the TTL accordingly. Modern routers almost always forward an IPv4 packet in less than a second and are required by RFC 791 to decrement the TTL by at least one. Therefore, the TTL becomes a maximum link count with the value set by the sending node. When the TTL equals 0,an ICMP Time Expired message is sent to the source IPv4 address and the packet is discarded.
Protocol Identifies the upper layer protocol. The size of this field is 8 bits. For example, TCP uses a Protocol of 6, UDP uses a Protocol of 17, and ICMP uses a Protocol of 1. The Protocol field is used to demultiplex an IPv4 packet to the upper layer protocol.
Header Checksum Provides a checksum on the IPv4 header only. The size of this field is 16 bits. The IPv4 payload is not included in the checksum calculation as the IPv4 payload and usually contains its own checksum. Each IPv4 node that receives IPv4 packets verifies the IPv4 header checksum and silently discards the IPv4 packet if checksum verification fails. When a router forwards an IPv4 packet, it must decrement the TTL. Therefore, the Header Checksum is recomputed at each hop between source and destination.
Source Address Stores the IPv4 address of the originating host. The size of this field is 32 bits.
Destination Address Stores the IPv4 address of the destination host. The size of this field is 32 bits.
Options Stores one or more IPv4 options. The size of this field is a multiple of 32 bits. If the IPv4 option or options do not use all 32 bits, padding options must be added so that the IPv4 header is an integral number of 4-byte blocks that can be indicated by the Internet Header Length field.