Basic Network Troubleshooting

Things you need to know before you can ask an intelligent question correctly:

Every beginner who comes here asks questions without providing the information necessary to answer that question. If you read and understand this tutorial, you will know how to ask a question properly. But you may not need to, because you may find the solution to your problem here.

Definitions:

• LAN: Local Area Network. Your local network
• WAN: Wide Area Network. The larger network your LAN connects to. The first step on the path to the Internet.
• DHCP: Dynamic Host Configuration Protocol. A server that assigns IP Address, Subnet Mask, Default Gateway and DNS Server values to your TCP-IP system.
• DNS: Domain Name System. DNS is what translates the Google URL into googles IP address. Like an automated phone book.
• Router: A network device with two interfaces; one side WAN, the other side LAN. They must be on different networks, or there is nothing for the router to do. The router inspects network taffic and decides: Local ( PC to printer ) or Long Distance ( PC to Internet ) and sends the traffic out the appropriate interface.
• Bridge, Wireless Bridge, Client: A device with one wireless and one wired network interface that connects wirelessly to an AP or Wireless Router. A bridge does the same thing as a USB WiFi adapter, or a wireless PCI card. The difference is that the bridge has an ethernet connector that allows the output to be shared by all of the computers on a wired network. A printer, scanner or DVR ( digital video recorder ) on a wired network with a bridge is effectively a wireless device.
• Protocol: The true and proper meaning of protocol is:
  
  Rules for communicating
  
  Unfortunately, this meaning has been corrupted into a dozen different sub-definitions.
• TCP/IP: A protocol used with Ethernet networks. Ethernet is the hardware, TCP/IP is a suite of protocols that defines the connection method and sets the parameters of the networks

TCP/IP Settings:

You have 4 values to deal with in TCP - IP:

IP Address. Think phone number. A unique identification number for each network interface in your system.

Subnet Mask. Works with IP address.

• Must be the same on all equipment in your network.
• The subnet mask decrees that a range of similar IP addresses are part of the same network.
• IP address out of the range of the Subnet mask = different network.
• Different subnet mask = different network.

If IP Address and Subnet Mask are good, you get local connectivity; your in house network equipment communicates.

Default Gateway: Think Dial 9 for an outside line. The default gateway is an IP address on your network that is the LAN side of a router that connects to a bigger network. The other side of that router is the WAN side of that router.

DNS Server. The address of a DNS server. Frequently multiple choice. One of these choices should be the IP address of your ISP. 

If you can connect to your printer but not to the internet, IP and Subnet Mask are good, Default Gateway or DNS Server may be bad.

Router TCP/IP settings: The router is the interface between two networks. Some of these settings affect the LAN the router serves.

IP Address: A router must have at least two IP addresses.

• The WAN address points to the internet connection.
• The LAN address is the internet connection for the local network
• The router LAN address is the Default Gateway for the local network devices

Subnet Mask: No effect on LAN
Default Gateway: No effect on LAN, but useful in troubleshooting
DNS Server: You can use the routers DNS Server IP in your LAN settings, or you can use the routers IP address.

Troubleshooting connections that go beyond your network:

If you can connect to your printer but not to the internet:

IP address is good
Subnet Mask is good
Default Gateway may be the problem

How to determine if Default Gateway is good: trace route to a random IP address. It does not matter if there is no such address; traceroute will document the steps the system makes to try to find the address. This gives us the information we need.

• If you can trace an IP address off your network the trace is going through your default gateway.
• If you can not trace an IP address off your network the trace is not going through your default gateway.

You may have the wrong value for the Default Gateway, the device may have a problem or the connections upstream from the device may have a problem.

How to determine if DNS Server is good: trace route to a known good URL.

• If you can trace an an IP off your network but not an URL the DNS server is not configured correctly or not working correctly.