This is a different application of random numbers. If you create a TCP connection, the server reply contains a sequence number. You have to know that number to

This is a different application of random numbers. If you create a TCP connection, the server reply contains a sequence number. You have to know that number to establish the connection, so you can’t establish the connection unless you are able to receive the reply. If however you can predict the number, you can create a connection without being able to receive packets from the server. So if for instance an administrator has setup their machine to accept connections to an insecure service only from a specific IP address, you can now set up such a connection without actually controlling the IP address.