At which OSI layer is a source IP address added to a PDU during encapsulation?

• A. Network Layer
• B. Data Link Layer
• C. Transport Layer
• D. Application Layer

Answer: (A)

The main idea is encapsulation and where logical addressing happens. IP addresses are used to route packets across networks, so they are added by the Network layer as data is encapsulated into an IP packet. When the Transport layer hands off its segment, the Network layer creates an IP header and assigns the source and destination IP addresses, producing an IP packet. Then the Data Link layer adds local addressing (MAC addresses) to form a frame for the local hop. The Application layer doesn’t deal with addressing for routing, and the Transport layer handles ports, not IP addresses. So the source IP address is added at the Network layer.