Perhaps I am wrong, but I think that networks have to know which port a datagram is sent to. But they are scattered in the IP-level and its header only contains the internet address of the destination computer. The destination port is in the TCP-header. So, how can a network know which destination port is relative with the datas that it is to transport (for instance datas destinated to a port 25 need less priority than datas destinated to a port 23).
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