Incorrect VLAN IP Subnets Configurations
On a switched network, each VLAN must belong to a unique IP subnet. If two devices in the same VLAN have different subnet addresses, they cannot communicate. Incorrect configuration is a common problem, and can be solved easily by identifying and changing the subnet address of the device causing the problem to the correct address.
In this example, the user of computer PC3 cannot connect to the WEB/TFTP server shown in the diagram.
In the diagram above, a closer examination of the IP configuration settings of PC3 shows the most common error in configuring VLANs and other IP networks: an incorrectly configured IP address. The PC3 computer is configured with an IP address of 192.168.10.21, but it should have been configured with 192.168.1.21.
PC3>ipconfig
IP Address…………………….: 192.168.10.21
Subnet Mask…………………..: 255.255.255.0
Default Gateway………………: 0.0.0.0
PC3>
Solution
After the IP configuration is changed to a correct address, pinging the server from PC3 computer reveals that it has regained connectivity to the WEB/TFTP server found at IP address 172.17.1.30.
PC3>ping 192.168.1.30
Pinging 192.168.1.30 with 32 bytes of data
…
Reply from 192.168.1.30: bytes-32 times-147ms TT-128
Reply from 192.168.1.30: bytes-32 times-147ms TT-128
Reply from 192.168.1.30: bytes-32 times-147ms TT-128
Reply from 192.168.1.30: bytes-32 times-147ms TT-128
