IP fragmentation and Path MTU

IP fragmentation and Path MTU concern packet delivery along the transport or network path. Separate the local socket, IP delivery, TCP or UDP behaviour, firewall filtering and NAT. A successful ping does not prove a TCP port is reachable, and an open port does not prove the application protocol is healthy.

How to find where the connection stops

Work from near to far: process and bind address → host firewall → route → NAT/port forwarding → CGNAT or ISP filtering → external test. For packet-size failures, consider MTU and Path MTU Discovery separately. Always interpret a port number together with its transport protocol.

The detail that matters

IPv4 routers may fragment in some circumstances, while IPv6 routers do not fragment transit packets. Path MTU Discovery depends on ICMP feedback such as Packet Too Big; blocking that feedback can create connections that work for small packets and stall for larger ones. To check the connection from outside your local setup, use 2ip ping and traceroute.