VLANs are a way of virtually dividing up a network into many different subnetworks, also referred to as segmenting. Each segment will have its own broadcast domain and be isolated from other VLANs.
On FreeBSD, VLANs must be supported by the network card driver. To see which drivers support vlans, refer to the vlan(4) manual page.
When configuring a VLAN, a couple pieces of information must be known. First, which network interface? Second, what is the VLAN tag?
See how the interface name includes the NIC driver name and the VLAN tag, separated by a period? This is a best practice to make maintaining the VLAN configuration easy when many VLANs are present on a machine.
Additional VLANs may be added, by simply adding the tag to the vlans_em0 field and adding an additional line configuring the network on that VLAN tag's interface.
It is useful to assign a symbolic name to an interface so that when the associated hardware is changed, only a few configuration variables need to be updated. For example, security cameras need to be run over VLAN 1 on em0. Later, if the em0 card is replaced with a card that uses the ixgb(4) driver, all references to em0.1 will not have to change to ixgb0.1.