TCP/IP Basics Overview
Hewlett-Packard Company Virtual Private Networking Concepts Guide
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implies that the default gateway's IP address must be on the
same subnet as the originating device. Computers can directly
communicate only with devices on their local subnet (as defined
by their IP address and subnet mask).
Default gateways are what make the Internet work. When a
packet is created by a desktop computer destined for an address
on the Internet, the desktop computer often sends the packet to
its default gateway. The default gateway is often an edge router
connecting the LAN (on which the desktop computer is sitting)
to the Internet. The edge router probably does not have specific
routes telling it what to do with the packet. The edge router,
therefore, most likely sends the packet off to its default gateway.
This cycle occurs until the packet arrives at a device that knows
where to find the destination address.
Application Port
Application Port
Application Port
Application Port
When a computer (or any network device) receives a packet, the
computer decides what to do with it. The computer may have
many different programs running simultaneously (for example, a
mail server and a Web server). Each program expecting to
receive or send packets from or to a network opens something
called a socket. If you look at an IP address as a street address
that identifies a building, then an open socket can be compared
to a room number within the building. The number given to a
socket is called an application port number.
Each packet contains both a source application port and
destination application port in its header. The destination
application port number is used by the receiving computer to
decide which program should be given the payload of the packet
for final processing.
Many application port numbers are standard. Some common
numbers are port 80, which is associated with http (www)
packets; port 25, which is associated with SMTP mail; port 110
(POP3 mail); port 23 (Telnet); and port 21 (FTP). Therefore,
when Web servers start, they usually connect to port 80 and
listen for requests to come in. Note that a Web server can be
configured to listen on another port, but most follow the
standard.
Packets with the source and destination application ports set to
2233 are encrypted with a HP VPN device.
