VLANs define broadcast domains in a Layer 2 network. A switched environment offers the technology to overcome flat network limitations. Switched networks can be subdivided into virtual networks (into different broadcast domains). The VLAN configuration is stored in the vlan.dat file, which is stored in nonvolatile memory.

If you are interested in security, you can go for CCNA(Sec). If you are interested in R&S, you can go for CCNP(R&S).