The Moteino transmission packet has three fields that are directly related to the 'points' in a link, and one 'field' that is indirectly defines to the link.
There was a good diagram of this floating around on the lowpowerlab site, but I can't seem to find it any more.
There is a good description of the network packet at:
http://lowpowerlab.com/blog/2013/06/20/rfm69-library/The three direct fields are: Network ID, Sender ID, Target ID. These IDs are 8 bits long and support a direct Point to Point connection, each field providing 254 choices (255 is 'special'). A node defines its own target, and there is nothing to prevent multiple
receiving nodes from using the same address so this supports one to one and one to many. The 'special' targets ID is 255 which is a Broadcast address and this supports one to ALL (on the same 'network'). Note, when a target responds, only one target with a duplicate node ID can transmit at a time, otherwise, as Eloy explained, collisions can occur.
The 'indirect field' is used to optionally control encryption of the data. It's not really a field in the packet, but it controls how the transmitted packet is encoded, consequently it is another means to control whether other nodes 'see' the packet or not.
So, to summarize, any network has two or more nodes. Senders use their ID to send to one or more Target nodes in the network. You have 255 potential networks and 255 potentially unique nodes. You can send one to one, or one to many. You just can't send many to one AT THE SAME TIME. You can have a topology with one central node and many nodes that send to the central one, but they need to send at unique points in time. The protocol has collision detection and 'some' limited correction in the form of retries.
Tom