You may see a number of symptoms occurring when there are problems with IP addressing, the subnet mask, or the gateway, including the following:
- Network devices can communicate within their local subnet but are unable to communicate with devices outside the local network. This usually indicates that there is some type of issue with the gateway configuration or operation.
- Not having any type of IP communication (internal or remote). This usually points to a major type of issue that might involve a lack of functionality on certain devices.
- Being able to communicate with only some of the IP addresses available. This is usually the most difficult problem to troubleshoot because it has many possible causes.
One of the first things you should always do during the troubleshooting process for such situations is double-check the IP address, subnet mask, and default gateway configuration on your devices. You should also check the documentation to verify this information.
If you are installing network devices for the first time, very often you will manually type in the IP address information, along with the subnet mask and default gateway. The recommendation is to verify this information before submitting it because mistakes happen often.
If you need to gather information during the troubleshooting process, you might want to perform packet capturing to see exactly which packets are being sent between devices. If you see packets from hosts that are not on your network, you may have some kind of VLAN misconfiguration issue. If you suspect that your subnet mask is wrong, you should check the parameters on other devices on your network. If the other machines work properly, you should use the same subnet mask on the device that is not working as expected and start testing again.
When using dynamic IP addressing (DHCP) to allocate IP address information, including subnet mask and default gateway, to devices in your network, you should investigate the DHCP server configuration because that might be another point where problems occur. The DHCP server could be misconfigured or the DHCP service could be jammed, so you should include this step in your troubleshooting process.
Other useful troubleshooting tools are traceroute and ping, which can help you identify the point in the network where problems are occurring.