The number in the socket column is simply a list index. The last number in the list indicates the total number of simultaneous Modbus TCP connections supported while the gateway is functioning as a Modbus RTU slave (or as multiple RTU slaves).

The first and last slave address determine which address(es) the gateway will respond to as an RTU slave, and these are routed to the IP address on this line. If first and last are the same number, then only this one slave address will be routed to the IP address given. If a range is given, all of these addresses will be routed to the IP address given, starting with outgoing slave address as indicated. If first incoming and first outgoing are the same, there will be no slave address translation. If they are different, outgoing slave addresses will be translated accordingly.

Connection status will show a non-zero error code if there is a socket error. Possible errors include:

70 Internal socket error
71 Modbus packet corrupt - protocol error
104 Connection reset by peer
111 Connection refused
112 Port already in use
113 Connection aborted
114 Network is unreachable
116 Connection timed out
118 Host is unreachable
119 Connection in progress (means unsuccessful connect attempt, still trying)