IPv4 Header – Header Checksum Field (16 bits)

Definition

• The Header Checksum field helps devices verify that the IPv4 header was not corrupted during transmission.
• Every router and the destination check this value before using the header. If a router changes the header (such as decreasing TTL), it calculates a new checksum.

What Problem It Solves

• Network interference or hardware issues can change bits in the IPv4 header.
• The Header Checksum helps detect these errors so devices don't make routing decisions using incorrect header information.

What Happens If Not Used

• A corrupted destination IP, TTL, or other header field might go unnoticed.
• Routers could forward the packet to the wrong destination or process it incorrectly.

Easy Wording

It checks whether the IPv4 header was damaged while traveling through the network.

Layman Example

Imagine a shipping label with a security seal. Every warehouse checks the seal before reading the delivery address. If the seal is broken, the package is rejected.

Technical Example

Request Flow:

• Your browser sends an HTTPS request.
• Your computer creates the IPv4 header and calculates its checksum.
• Router A verifies the checksum, decreases TTL by 1, recalculates the checksum, and forwards the packet.
• The web server verifies the checksum before reading the destination IP and Protocol fields.

Limitation: The Header Checksum protects only the IPv4 header, not the actual website data. Source IP Address and Destination IP Address, identify who sent the packet and where it should go.