Virtual LANs or VLANs are virtual separations within a switch that provide distinct logical LANs that each behave if there were configured on a separate physical switch. Some Important points about VLANS.

•  All the servers connected with in a VLAN will have same broadcasting domain, and can connect to each other directly
•  Servers belongs to different VLANs has to communicate with each other using external Routing device.  
•  To Expand the VLAN across multiple physical switches, physical switches needs to be interconnected using links called Trunks

 How VLAN knowledge Hhlps for System Administration?

The majority of the problems that a system administrator experience,  in his daily job,  are related to network Connectity. The troubleshooting procedure for Network Connectivity issues related to any server starts with two basic questions, and they are

  -  Whether the server able to connect with the servers from same VLAN?

  -  Whether the server able to connect with the Servers from Different VLAN?

Based on the answers from this question, the system administrator further troubleshoots the problem interms of cabling issues, switch connection issues,  IP related issues or gateway related issues.

How to identify the VLAN configured for a server network interface?

The VLAN configuration happens at Switch port side, and it is completely invisible for System administrators. But Still system admin can identify the server network segment by using the network tracing utilities to look at the network packets receieved by the the network interface.

some Sample tools that can be used to identify network traffic are  snoop utility Solaris, tcpdump for linux ..etc. (  if you want to know about the usage of snoop / tcpdump, please refer the post “Network physcial connectivity Checks for Solaris and Linux“  )