When connecting to a real network using other connection types than port forwarding, Simics needs low-level access to the simulation host's Ethernet interfaces to send and receive packets. However, operating systems do not usually allow user programs low-level access to Ethernet interfaces: it requires specific configuration with administrative rights.
The access method used by Simics is called TAP; it is a virtual Ethernet interface on the simulation host. Ethernet bridging and host connections require the use of TAP, but, as stated above, port forwarding connections do not.