ESX servers have a whole virtual network within them: guest machines connect to virtual switches and uplinks from these switches to the outside. Although, one term that is not used outside of ESX are "port groups." After reading a bit about them and looking at the various tools the the ESX console provides, I think the best way for a network engineer to understand port groups is to see them as network hubs connected to a single vswitch port. This actually makes sense for multiple reasons:
- All members of a port group share common attributes like a VLAN tag
- All members of a port group can see all of the packets sent by other members of this port group
- A port group is always connected to a single vswitch
Port groups in ESX are identified by their name, which must be unique within an ESX server. Having the same port group names in different ESX servers, however, makes a lot of sense, especially when moving guests around between them. More on this later.