Question

Which layer 4 field is responsible for guiding a segment to the correct application on a destination host?

Answer 1

Destination port  is the field responsible for guiding a segment to the correct application on the destination host.

Destination ports may be "well-known ports" (0-1023) for the major Internet applications, such as Web and email. For example, all port 80 packets (HTTP packets) are directed to and processed by a Web server. User "registered ports" (1024-49151) are assigned to applications that are mostly vendor specific, such as Skype and BitTorrent.

Some might think that destination port and source port can be used interchangeably.

However, in contrast, a source port is a next-available number assigned by TCP/IP to the user's machine. This assigned number is how the network address translation (NAT) determines which user to send back the responses to (see NAT). Although that same client number may be used simultaneously within thousands of organizations, each TCP/IP network keeps track of its own assigned numbers for internal use only. A "socket" is the combination of port number and IP address