IPv4header

The IPv4 header is the portion of an IPv4 packet that contains routing and delivery information. It precedes the payload and, together with the payload, determines how the packet is processed and forwarded by networks. The header has a minimum size of 20 bytes and can be extended up to 60 bytes with optional fields. The total length field, which is 16 bits, specifies the entire packet size in bytes, including header and data, and thus the maximum packet size is 65,535 bytes.

Key fields and their purposes include the version and Internet Header Length (IHL). Version is a 4-bit field that identifies the protocol as IPv4, while IHL is a 4-bit field indicating the header length in 32-bit words. The Type of Service field, now commonly used as Differentiated Services Code Point (DSCP) and Explicit Congestion Notification (ECN), conveys handling priority and congestion information. The Total Length field encodes the overall length of the packet. Identification, Flags, and Fragment Offset participate in fragmentation and reassembly when packets traverse networks with differing maximum transmission units. Flags include the Don’t Fragment and More Fragments indicators, and Fragment Offset specifies the position of a fragment within the original datagram.

The Time to Live (TTL) field limits a packet’s lifespan to prevent looping, while Protocol identifies the upper-layer protocol (for example, TCP, UDP, ICMP). The Header Checksum provides error detection for the header only and is recalculated when header fields change en route, such as TTL decrement. Source and Destination Address fields carry the 32-bit IP addresses of the sender and recipient. Options and padding, if present, extend the header length to ensure alignment on 32-bit boundaries.

In summary, the IPv4 header encodes essential addressing, routing, and control information that guides packet delivery and reassembly.