IPv4 Header – Source IP Address Field (32 bits)

Definition

• The Source IP Address field contains the IPv4 address of the device that created the packet.
• It identifies the sender so the destination knows where the packet came from and where replies should be sent.

What Problem It Solves

• Communication on a network is two-way. After receiving a request, the server needs to know who sent it so it can send a response back.
• The Source IP provides the sender's network identity throughout the packet's journey.

What Happens If Not Used

• A web server could receive your request but would have no address to send the response to.
• Web browsing, API calls, and most Internet communication would fail.

Easy Wording

It tells the receiver who sent the packet.

Layman Example

Think of sending a letter with your return address on the envelope. If the recipient wants to reply, they use that address.

Technical Example

Request Flow:

• Your browser requests https://example.com.
• Your router may use NAT to replace your private IP with your public IP.
• The IPv4 packet travels through multiple routers.
• The web server reads the Source IP Address and knows where to send the HTTP response.
• The response follows the network back to your router, which uses NAT to deliver it to your computer.

Limitation: The Source IP identifies who sent the packet, but it doesn't specify who should receive it. Destination IP Address, identifies the intended recipient.