When the Lync Server 2010 edge server is not configured with two network interfaces or both the network interfaces are not configured with IP addresses from different subnets. Some or all of the following functionality listed below might not work as expected.
External User Access
Public IM connectivity
Push notification for Lync 2010 mobility client
Incorrectly configured Lync Server 2010 edge server network interfaces
The Lync Server 2010 edge server should have two network interfaces that are configured with IP addresses from different subnets.
For more information on setting network interfaces for Lync Server 2010 edge server role, please refer the following document Microsoft TechNet document: