IP Packet Header

In 2025, the IPv4 packet header remains widely used despite IPv6 adoption, supporting legacy systems and critical applications like industrial IoT, where its compact structure is advantageous in constrained environments. The network layer converts transport layer segments into packets, with the IPv4 packet encapsulating the data unit and adding header information. In 2025, this process supports critical legacy systems alongside emerging technologies.

Important fields in the IPv4 packet header include

• Version – IPv4 packet header contains a 4-bit binary value set to 0100, identifying this as an IP version 4 packet.
• Internet Header Length (IHL)—Internet Header Length (IHL) is a 4-bit field indicating the header length in 32-bit increments, with a minimum of 20 bytes and a maximum of 60 bytes. In 2025, optimizing IHL is key for efficient packet processing in high-speed networks.
• Differentiated Services (DS) – Previously called the Type of Service (ToS) field, the DS field is an 8-bit field used to decide the priority of each packet.
• Differentiated Services Code Point (DSCP)– Usually set to 0, but may indicate particular Quality of Service needs from the network; the DSCP defines how routers should queue packets while waiting to be forwarded.
• ECN: Explicit Congestion Notification. It carries information about the congestion seen in the route.
• Total Length: Length of entire IP Packet (including IP header and IP Payload).
• Identification: If an IP packet is fragmented during transmission, all the fragments contain the same identification number. To find the original IP packet they belong to.
• Flags: As required by the network resources, if an IPv4 Packet is too large to handle, these ‘flags’ tell if it is fragmented or not. In this 3-bit flag, the MSB is always set to ‘0’.
• Fragment Offset: This 13-bit field specifies the fragment’s place in the original fragmented IP packet.
• Time-to-Live (TTL): Time-to-Live (TTL) is an 8-bit field limiting a packet’s lifetime, decrementing by one per router hop. If it reaches zero, the packet is discarded with an ICMP message. In 2025, TTL aids in managing latency in 5G networks.
• Protocol – This 8-bit binary value indicates the data payload type that the packet is carrying, which enables the network layer to pass the data to the appropriate upper-layer protocol. Common values include ICMP (1), TCP (6), and UDP (17).
• Header Checksum: This 16-bit field stores a checksum of the header. The receiver can use the checksum to check for errors in the header.
• Source IP Address – Contains a 32-bit binary value that represents the source IP address of the packet.
• Destination IP Address – Contains a 32-bit binary value that represents the destination IPv4 address of the packet.
• Options: This is an optional field of the IPv4 packet header, which is used if the value of IHL is greater than 5. These options may include security, record route, timestamp, etc.
• As of 2025, fields like DSCP and ECN are increasingly leveraged in 5G networks for Quality of Service (QoS) and congestion control, optimizing performance in high-traffic scenarios.

IPv4 Header Evolution and Limitations

The IPv4 header, with its 20-60 byte length, was designed for early internet needs but faces limitations in 2025, such as address exhaustion (mitigated by NAT) and limited options for security. Advances like header compression techniques are being explored to enhance efficiency in 400 Gbps networks.

The two most common fields of the IPv4 packet header are the source and destination IP addresses. These fields find the source and destination of the packet. Typically, these addresses do not change while travelling from the source to the destination.