How do I further diagnose TCP connectivity (timeout) problems? (ICMP, DNS work fine) I have two Macs behind a cable modem/WAP. One works fine. The other is able to ping servers but is unable to telnet to them. For instance pinging what.org works fine but I get operation timed out errors when I wget what.org or telnet what.org 80. And nmap what.org -Pn says host is up but all ports are filtered. I'm pretty sure this is a problem with the computer, not with the modem/router. Also, strangely some of my Chrome tabs can still reload, duplicate, and function, like google, Gcal and Gmail, but other's can't, and all new domains fail with Error 118 (net::ERR_CONNECTION_TIMED_OUT): The operation times out.. Skype chat and dropbox work. How can I further diagnose this problem?
First step: check that Mac n°2 is connected with the network interface you think it should (i.e. Ethernet and not AirPort or anything else).

If you are using the famous Automatic network configuration you are never sure on which interface your computer is talking or replying.
For example, your Mac n°2 might well be plugged toward the Internet through your neighbour Wi-Fi connection.