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routing > "The network can not be reached" [sic]

From the generalnetworkerror.com Search archives, comes this query about the network not being reachable. Numerous possibilities could be the root cause. Without further details, knowing where to start to troubleshoot is difficult, so we'll start from the square one.

Start with a traceroute (tracert in Windows) from the command (CMD) prompt. Assuming the routers and firewalls you traverse from source to destination are configured to respond to traceroutes (which incidentally could be done with either icmp echo'ss to icmp traceroute's), see which hop is last reported.

If hop 1 times-out, check your default gateway which, of course, must be on the same subnet (ip address & subnet mask) as your own machine. I'm assuming you don't have any layer 2 (L2) issues here with reachability to your gateway.

Keep in mind that the network is a two-way medium, so in order for you to reach any host on the network requires that the host can reach you. Having packets make it to your host doesn't guarantee a path back. Take, for example, the simple case of a server not having a default gateway configure correctly or at all. Packets may reach that server, but then the server has no idea how to get back to you.

Firewalls could be in your path stopping access as well. With a tool like wireshark[.org], you can see if the 3-way handshake is being completed for TCP. If you initiate a connection towards the server, you'll see a TCP packet with a SYN flag sent. This should be returned from the server with a SYN ACK. And finally, an ACK is sent from your side to the server. All this must work before an application can send any meaningful data such as a browser sending a HTTP GET request to a web server.

Dec 6, 2012 at 1:26 | Registered Commentergeneralnetworkerror