How to determine use which virtual IP through NEIPv4Route?

When I create NEIPv4Settings, I add two IPs. And I set serveral NEIPv4Route in includedRoutes.

My question is how can I choose use which virtual ip as my source destination in my packet.

The source destination in my packet always is the first IP in NEIPv4Settins's addresses array.

NEIPv4Settings *ipv4Settings = [[NEIPv4Settings alloc] initWithAddresses:@[@"111.111.111.11", @"222.222.222.22"]
                                                                 subnetMasks:@[@"255.255.255.0", @"255.255.255.0"]];

NEIPv4Route *route = [NEIPv4Route defaultRoute];
route.gatewayAddress = @"222.222.222.1";
ipv4Setting.incluedRoutes = @[route];

networkSettings.IPv4Settings = ipv4Settings;