The destination IP is where that packet is going.

And then the destination port is like the destination IP.

You know what the destination port and the destination IP is.

And what you're hoping for is that the source port will be the same, even if the destination IP is different.

We know that four bytes, 32 bits, is for the destination IP.

Firstly the original destination IP and port must somehow be communicated to the proxy.

It's got to be the source and destination IP.

The destination IP gets it to you.

It's called the Land attack, where the source and the destination IP are both the same as that machine's.

So a sender who puts a packet onto the Internet, aimed at a destination IP, doesn't know what's going to happen.