he header is read left to right, top to bottom. Top left being the first bit and bottom right being the last bit in the header.  * **Version**: Identifies the version of IP used. The IPv4 and IPv6 headers have different structures. * Length of 4 bits * IPv4 = 4 (0100) * IPv6 = 6 (0110) * **IHL (Internet Header Length)**: The final field of the IPv4 header (Options) is variable in length, so this field is necessary to indicate the total length of the header. * Length of 4 bits. * Identifies the length of the header in 4-byte increments. A value of 5 means the size is 20 bytes. * Minimum IPv4 header length is 20 (value 5) bytes with empty Options field. * Maximum IPv4 header length is 60 (value 15) bytes with maximum length Options field. * The maximum size of the IP Options field is 40 bytes * **DSCP (Differentiated Services Code Point)**: * Length of 6 bits. * Used for QoS (Quality of Service). * Used to prioritize delay-sensitive data (streaming voice, video, etc). * **ECN (Explicit Congestion Notification)**: Provides end-to-end (between two points) notifications of network congestion without dropping packets. * Length of 2 bits. * Without ECN, congestion in a network is usually signaled by dropping packets. * It's an optional feature that requires both endpoints, as well as the underlying network infrastructure to support it. * **Total Length**: Indicates the total length of the packet (L3 header + L4 segment). * Length of 16 bits. * Measured in bytes. * Minimum value of 20 bytes (IPv4 header with no Options and no encapsulated data). * Maximum value of 65,535 bytes (maximum 16-bit value). * **Identification**: If a packet is fragmented due to being too large, this field is used to identify which packet the fragment belongs to. * Length of 16 bits. * All fragments of the same packet will have their own IPv4 header with the same value in this field. * Packets are fragmented if larger than the MTU (Maximum Transmission Unit), which is 1500 bytes. * Fragments are reassembled by the receiving host. * **Flags**: Used to control and identify fragments. * Length of 3 bits. * Bit 0 is reserved, always set to 0. * Bit 1 is the don't fragment (DF bit), used to indicate a packet that should not be fragmented. * Bit 2 is the more fragments (MF bit), set to 1 to indicate more fragments in the packet and 0 to indicate the last fragment. * Unfragmented packets will always have their MF bit set to 0 since there are no fragments. * Bit of 0 means `Not set` * Bit of 1 means `Set` * **Fragment Offset Field**: Indicate the position of the fragment within the original, unfragmented IP packet. * Length of 13 bits * Allows fragmented packets to be reassembled even if the fragments arrive out of order. * **Time To Live**: A router will drop a packet with a TTL of 0. * Length of 8 bits. * Used to prevent infinite loops, avoiding heavy traffic congestion. * Originally designed to indicate the packet's maximum lifetime in seconds. * In practice, indicates a 'hop count': Each time the packet arrives at a router, the router decreases the TTL by 1. * The recommended default TTL is 64. * **Protocol**: Indicates the protocol of the encapsulated L4 PDU. * Length of 8 bits. * Value of 6 indicate TCP. * Value of 17 indicate UDP. * Value of 1 indicate ICMP. * Value of 89 indicate OSPF (dynamic routing protocol). * **Header Checksum**: A calculated checksum used to check for errors in the IPv4 header. * Length of 16 bits. * When a router receives a packet, it calculates the checksum of the header and compares it to the one in this field of the header. * If they do not match, the router drops the packet. * Used to check for errors only in the IPv4 header. * IP relies on the encapsulated protocol to detect errors in the encapsulated data. * Both TCP and UDP have their own checksum fields to detect errors in the encapsulated data. * **Source IP Address**: IPv4 address of the sender of the packet. * Length of 4 bytes (32 bits). * **Destination IP Address**: IPv4 address of the intended receiver of the packet. * Length of 4 bytes (32 bits). * **Options**: Optional field. * Can be 0 - 40 bytes (0 - 320 bits) in length * Rarely used. * If the IHL field is greater than 5, it means that Options are present.