This week I have been looking at how to detect TCP/IP ports on a local area network to see if an application was setup correctly. And while using ping and tracert from a DOS prompt works well for somethings, I was looking at what else I could use. For Mac OS X users, the Network Utility tool features ping and traceroute options, or you can always use a terminal window directly to ping and traceroute. To verify that you have a listening port, the command line utility netstat -a comes in handy on both Windows and Mac OS X. With this you can verify that a particular port number is listening. You can still use the website CanYouSeeMe.org, if you want to verify that your particular computer is open to the Internet, but this only works if your router is setup to forward requests for this port. In the case where you want to run a VNC or some other service for only local network connections, netstat works much better to verify that the port is in fact listening.
If you read my previous post on Troubleshooting Ports on DSL, then perhaps you will get the whole idea on how to forward ports, but as I discovered, powering off your routers may cause packet loss afterwards.
In a typical DSL setup your main DSL modem provide DHCP services, and although there is only one physical ethernet port that it connects to, unless you specifically setup the DSL modem to forward all traffic to your LAN router, chances are a reset could send packets to another ip address and all of a sudden your packets get lost going to a different ip.
To work around this, you either have to be careful about how you reset your routers or force the DSL modem to route everything to one IP address, therefore losing some of the firewall protection that the DSL modem is providing.
Here is a proper way to reset your home network and routers.
1. Turn off everything. This includes computers, routers, DSL modem.
2. Power on the DSL Modem first. Wait a few minutes until the LAN and WAN lights turn solid, usually green.
3. Power on your home router, the Linksys, Netgear, D-Link, or whatever brand you have.
4. After a couple of minutes, turn on the computers and other devices on your network.
5. Verify the NAT on the DSL modem is still setup properly and that the home router is connecting to the DSL modem with the correct IP address.
For the Cisco 678 ADSL modem, telnet into it and type:
The last lines that come up will show the last packets that were sent and what local ip they were sent to. It should be 10.0.0.2 for the 678, unless you changed it.