Go to Analyze > Select Subnets 
Detects which part of a network is a subnet. A subnet is defined as a part of the network that isn't connected to the rest of the network. The main net is defined as the part of the network, where node 1 is found. Node 1 is the one with the highest degree and if there are more than one of these, it is the West-most.
The following dialog is displayed:
If you click "Yes" then you may be prompted to add a result column to your table:
This will update the network table with the records that are in a subnet.
After Pressing "OK" the Browser is displayed. This are all the links found in a subnet:
Below is a map that shows a subnet identified by the function.
Please note that settings such as one-ways, avoiding no through roads, limits and avoids may affect the results. Therefore we recommend that this test is run without any of those setting being active.
(The OSM Sample data has no subnets)