IPv4 Header – Protocol Field (8 bits)

Definition

• The Protocol field tells the receiving device which Layer 4 (Transport Layer) protocol should receive the IPv4 payload.
• After IPv4 finishes processing the packet, it uses this field to hand the data to the correct protocol.

What Problem It Solves

• An IP packet can carry different types of data, such as a web request (TCP), a DNS query (UDP), or a ping (ICMP).
• The Protocol field ensures the payload is delivered to the correct protocol instead of guessing.

What Happens If Not Used

• The operating system would receive the IP packet but wouldn't know who should process the payload.
• Web browsing, DNS lookups, ping, and many other network functions would fail.

Easy Wording

It tells IPv4 which program inside the operating system should receive the packet next.

Layman Example

Think of an office mailroom. Every package has a department name like HR, Finance, or IT. The mailroom reads the label and sends the package to the correct department.

Technical Example

Request Flow:

• Your browser requests https://example.com.
• DNS first uses UDP (Protocol = 17) to find the server's IP address.
• Your computer then sends an HTTPS request using TCP (Protocol = 6).
• The web server receives the IPv4 packet, reads Protocol = 6, and delivers the payload to the TCP module.
• TCP processes the segment and passes the HTTP request to the web server application.

Limitation: The Protocol field identifies who should receive the payload, but it doesn't verify whether the IPv4 header was corrupted during transmission. Header Checksum, detects header errors.