When the problem is between systems rather than on one.
BASIC TESTS
ping tests whether a host responds at all traceroute shows the path and where it stops telnet or nc tests whether a specific port is reachable dig or nslookup tests DNS resolution
WORKING OUT WHERE THE FAULT IS
From your machine to the server: if ping fails from multiple networks, it is the server or its network. If it fails from one, it is that network or a block.
From the server outward: if the server cannot reach external services, check its own DNS configuration and outbound firewall rules.
DNS PROBLEMS
If the server cannot resolve hostnames, nothing that depends on external services works: package updates, API calls, mail delivery. Check the resolver configuration.
FIREWALL BLOCKS
A port closed in the firewall behaves like a service that is not running. Check both.
MTU AND FRAGMENTATION
Occasionally a connection establishes but large transfers hang. Rare, and worth knowing about because it is otherwise baffling.
WHEN TO INVOLVE US
If the fault appears to be in the network between the server and the internet, open a ticket with traceroute output from both directions.
