Which field helps determine whether a packet is from a valid source?

Which field helps determine whether a packet is from a valid source?

• A. Interface
• B. Protocol in use
• C. Source IP address
• D. TCP code bits

Answer: (C)

Packets carry an IP header that includes the source IP address, which identifies where the packet says it came from. This field is the primary indicator used to judge whether the sender is legitimate. If the source IP address aligns with what’s expected for the path or passes anti-spoofing checks (like ingress filtering or reverse path forwarding), the packet is treated as from a valid source. If the address is forged, the packet can be rejected even if it arrives at a legitimate interface. The other fields don’t establish origin in the same way. The interface tells you where the packet entered, not who sent it. The protocol in use merely indicates what kind of payload is carried, not the sender’s identity. TCP code bits show connection intent (like initiating or finishing a session), not where the packet originated.

Packets carry an IP header that includes the source IP address, which identifies where the packet says it came from. This field is the primary indicator used to judge whether the sender is legitimate. If the source IP address aligns with what’s expected for the path or passes anti-spoofing checks (like ingress filtering or reverse path forwarding), the packet is treated as from a valid source. If the address is forged, the packet can be rejected even if it arrives at a legitimate interface.

The other fields don’t establish origin in the same way. The interface tells you where the packet entered, not who sent it. The protocol in use merely indicates what kind of payload is carried, not the sender’s identity. TCP code bits show connection intent (like initiating or finishing a session), not where the packet originated.