Interface Number mentioned in Log File

A client connection to our FTP server has started to fail recently. While trying to find the cause, I noticed a small difference in the log:

Previously (successful connection):

CONNECT [610245] - Incoming connection request on FTP interface 16 at 192.168.18.220:21 accepted from ...

Currently (unsuccessful):

CONNECT [ 17621] - Incoming connection request on FTP interface 5 at 192.168.18.220:21 accepted from 

Is this interface number significant? I can't find any reference to what it is.

Thanks