Why Linux?

Most larger IP networks are based around managed switches and routers (Cisco, Extreme, Foundry...).  Support for IPv6 in these products is coming along, but is often only available in pre-release code. Personally, I prefer to stay away from this on core network equipment!

VLAN networks are Layer 2 (Ethernet) based. Logically, an ethernet network is a broadcast domain for ethernet frames. In a VLAN network, these broadcast domains are propogated between switches via "trunk" links. A Linux machine with a VLAN trunk can join as many of these broadcast domains as is permitted.