Review the protocol used for unreachable and TTL-exceeded notifications and connectivity diagnostics in an IP network.
Which description is correct?
ICMP carries IP-layer control, error, and diagnostic messages. It does not use application TCP or UDP port numbers.
Detailed explanation
Incorrect. A reliable byte stream is TCP's service.
Incorrect. This describes UDP transport and its port fields.
Correct. ICMP carries IP control and diagnostic messages such as echo requests and replies.
Incorrect. Time synchronization is NTP's role.
Try it yourself
An example you can run in a temporary verification environment.
python3 -c "import socket; print(socket.getprotobyname('icmp'))"Expected result
1（IPv4 ICMPのプロトコル番号）Key points
- IP control and error messages
- Echo request and reply
- No TCP/UDP ports
Notes
- Environment: Python 3 socket / 外部通信なし
- Command output formatting can vary slightly by distribution or tool version.
- Run the example in a temporary directory or process when possible.
Foundation review
Network prefix
The part of an IP address that identifies the network rather than a host.
Transport protocol
TCP and UDP provide different delivery, reliability, and connection semantics.
Service port
A numbered endpoint used to deliver traffic to a network service.