This is what Mercury help says about this subject (in this case MercuryS, but applies to other server modules as well):
If your computer supports multiple IP interfaces, you can use this field to tell MercuryS which interface it should select when listening for connections: enter the interface as a dotted IP address in the general form www.xxx.yyy.zzz. As an example, your system may have one IP address assigned to a dialup PPP connection, and another, different IP address assigned to a local Ethernet network - you would enter here the interface MercuryS should use. If you leave this field blank, MercuryS will listen on all available interfaces. Unless you are very sure of what you are doing, or have been instructed by an ISP or network administrator, you should leave this field blank. If you change the IP interface in this field, you must restart Mercury before the new interface number will be used.
<p>This is what Mercury help says about this subject (in this case MercuryS, but applies to other server modules as well):</p><blockquote><p><i>If your computer supports multiple IP interfaces, you can use this field to tell MercuryS which interface it should select when listening for connections: enter the interface as a dotted IP address in the general form www.xxx.yyy.zzz. As an example, your system may have one IP address assigned to a dialup PPP connection, and another, different IP address assigned to a local Ethernet network - you would enter here the interface MercuryS should use. If you leave this field blank, MercuryS will listen on all available interfaces. Unless you are very sure of what you are doing, or have been instructed by an ISP or network administrator, you should leave this field blank. If you change the IP interface in this field, you must restart Mercury before the new interface number will be used.&nbsp;</i></p></blockquote>