When port forwarding is done, it relates to the ip of that particular computer. A computer with a different ip address will not be able to use that same port forward rule because the id (ip address) doesn't match. This is most applicable on a network. Since all computers on a network will have a different address. eg: router address: 10.0.0.1, computer #1 address: 10.0.0.3, computer #2 address: 10.0.0.4, etc.