Packet received from multicast broadcast has external IP

Question

Hello,
I have a program set up to send a message over the network via multicast. When the message is received, it checks if the IP of the packet is equal to the local host's IP, and if it is it has different behaviour. This was working fine when working locally on the LAN, but when running it on a web-facing machine, the received packet's IP is being returned as its external Internet one, so I can't do the comparison to the local host's IP.
I don't have a ton of experience in this area so I don't know what is causing this to happen or how it can be fixed. Below is the excerpt from my code which compares the packet's IP to that of the local machine. Any insight would be appreciated, thanks in advance.
InetAddress address = packet.getAddress();
logger.debug(String.format("Received message from %s", address.getHostAddress()));
if (!(address.getHostAddress().equals(InetAddress.getLocalHost().getHostAddress())))
  logger.debug("Host address is not local address (" + InetAddress.getLocalHost().getHostAddress() + ")");
}

Answer 1

You need to enumerate all the NetworkInterfaces and all the IP addresses they represent, and check the incoming packet's source IP address against all of those.