Each peer is only aware of its immediate successor; for instance, peer 5 knows the IP address and identifier for peer 8 but does not essentially know anything about any other peers that may be in the DHT. This circular arrangement of the peers is a particular case of an overlay network. In an overlay network, the peers form an abstract logical network which resides above the "underlay" computer network consisting of physical links, routers, and hosts. The links in an overlay network are not physical links, but are simply virtual liaisons between pairs of peers. In the overlay in Figure 1(a), there are eight peers and eight overlay links; in the overlay in figure 1(b) there are eight peers and 16 overlay links. A single overlay link normally uses many physical links and physical routers in the underlay network.