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IP protocol number

The numeric identification of the upper layer protocol that an IP packet should be sent to. The number is stored in the header that is prefixed to an IP packet. Note that the IP protocol number is not the same as the port number (see TCP/IP port), which refers to a higher level, such as the application layer. Following are some of the common IP protocol numbers.

  Protocol   #   Purpose

  TCP        6   Transport type
  UDP       17   Transport type

  ICMP       1   Control message
  IGMP       2   Multicast

  EGP        8   Exterior gateway
  IGP        9   Interior gateway
  RSVP      46   Reserve bandwidth

  IP         4   Encapsulation (IP in IP)

TCP/IP port

A number assigned to user sessions and server applications in an IP network. Port numbers, which are standardized by the Internet Assigned Numbers Authority (IANA), reside in the header area of the packet being transmitted and thus identify the purpose of the packet (Web, email, voice call, video call, etc.).

Destination Ports Are Server Applications
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. See well-known port, port forwarding and opening a port.

Source Ports Are the User Sessions
The 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 (see Unix socket).

Reverse Numbers for the Trip Back
In the response from the server, the port numbers and IP addresses are reversed. The packet's destination port becomes the unique source port number assigned to that user's TCP/IP session.

We're Listening
TCP/IP servers are said to be "listening" for their port numbers to know when to accept incoming packets. If a human action had to be chosen for this, "looking" would have been more accurate. People "listen to sounds" but "look" for data; however, computerese has never been known for clarity. See TCP/IP and NAT.

Retrieving a Web Page
To request a Web page from the Internet, port 80 must be open in the network firewall to let the request go out. When sending back the Web page, the port numbers are reversed, and the user's network accepts the external packets because the request was initiated by a user. The IP is the numeric address of the source or destination network (see dot address).

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