Connecting Subnets

To correctly add multiple subnets in Sofros LAN Messenger, it is necessary to understand how each instance of the application builds its user list.

First, one instance of Softros LAN Messenger sends a UDP packet to the LAN broadcast address. It calculates the address based on the IP address and subnet mask of the computer it is running on. Any packet sent to a broadcast address is received by all LAN computers. After receiving the packet, other instances of Softros LAN Messenger running on the network respond with a TCP packet sent to the sender’s address. As a result, the sender instance receives the packets from the other instances and, based on their responses, builds its user list. This technique is used by all instances running on the network (sending one packet to the broadcast address and receiving responses from the other instances). Thus, Softros LAN Messenger works within one subnet without any additional configuration.

Softros LAN Messenger working within one subnet

The task of getting Softros LAN Messenger to work across multiple subnets involves delivering to another subnet that first packet sent to the broadcast address. Each subnet has a unique broadcast address. It is impossible for an instance from one subnet to calculate the broadcast address of another subnet without having any information about it. This is why the broadcast address of the other subnet needs to be added to said instance’s Broadcasts list. This way the instance will be able to send the packet, having received which, instances from the other subnet will be added to its user list.

Softros LAN Messenger working across two subnets

To add a subnet to the Broadcasts list, follow these steps.

  1. On the Network tab in the Settings dialog box, click Add.
  2. In the dialog box which appears, select an option serving best for your network (Single IP/broadcast address, Subnet info, IP address range), fill in the text boxes below depending on your selection, and then click Add.

  3. Adding a subnet to the Broadcasts list

For consistent operation, Softros LAN Messenger needs to be configured on both subnets; one subnet should add the other and vice versa.