Most high level applications do not deal with any of the low level details. They open a channel by specifying a hostname and a port number, and are given a reli

Most high level applications do not deal with any of the low level details. They open a channel by specifying a hostname and a port number, and are given a reliable bidirectional byte stream from the network layer. As far as most applications are concerned, the hostname is just a string that is interperated by the network layer. Be that through a DNS lookup, or parsing as an address native to the underlying network protocol. A minority of applications get fancy and request a datagram oriented link, which the network layer also provides (with an admittadly small limit of 65KB that leaks over from that. Few applications ever go deeper than that.