Understanding TCP Connection Failures: Nonexistent IP and Unavailable Port Scenarios
This article explains what happens when a client attempts a TCP connection to an IP address that does not exist or to a valid IP where the target port is not listening, covering ARP failures, SYN retransmissions, RST responses, and the importance of solid networking fundamentals for interview preparation.
Hello, I'm Xiao Lin. Today I share two ByteDance interview questions that focus on network scenarios.
Q1: What happens when a client connects to a non‑existent IP address?
The answer depends on whether the target IP is in the same LAN as the client.
Case 1 – Same LAN: The client sends an ARP request to resolve the MAC address of the target IP. Since the IP does not exist, no device replies, so the client cannot obtain the target MAC address and therefore cannot construct the Ethernet frame needed to send the SYN packet.
Case 2 – Different LAN: The client forwards the SYN packet to its default router. The router replies to the ARP request with its own MAC address, allowing the client to send the SYN. Because the target IP does not exist, the SYN never reaches a host and no ACK is received. The client eventually times out, retransmits the SYN several times, and after reaching the maximum retry count the connection is released.
Q2: What happens when a client connects to an existing IP address but the port is not listening?
The SYN packet reaches the target host successfully. The host’s kernel sees that the destination port has no listening process and responds with an RST packet. Upon receiving the RST, the client immediately releases the connection.
These conclusions illustrate why a systematic understanding of the OSI/TCP‑IP layers is essential for answering scenario‑based interview questions.
For a deeper dive into how a data packet traverses the network, see my previous article "Exploring the Journey of a Data Packet in the Network" (link provided in the original text).
Additional question (not answered here): If a client sends a UDP packet to a valid IP but an unavailable port, UDP does not generate an RST; the behavior differs from TCP.
Finally, remember that mastering network fundamentals will help you handle such interview scenarios with confidence.
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