I have geth 1.5.4 installed on my ubuntu box. Here is the geth command line documentation for networkid:
--networkid value Network identifier (integer, 0=Olympic (disused), 1=Frontier, 2=Morden (disused), 3=Ropsten) (default: 1)
So, to connect to the Ropsten network, I started geth with
geth --fast --networkid 3 console
However, it did not connect to the Ropsten network at all. My node started syncing but not sure what network it connected to. When I don't specify networkid but just pass --testnet:
geth --fast --testnet console
it connects to Ropsten network correctly and starts syncing.
What is the difference between specifying