Technical data

118 Theoretical and general applications www.westermo.com
Ports
An application receives data on a special port number that identifies communication
with this application.
For example, a computer can be both a web server, E-mail server and DNS server
running at the same time. In order for the traffic to the different applications not to
collide, it must be divided up, this is done by predefining the port number to the appli-
cation. Port numbers between 1 and 1024 are known port numbers and must not be
used by applications other than those specified.
Examples of known port numbers are:
21 ftp File transfer
23 Telnet Telnet
25 smtp Mail, Simple Mail transfer
80 http www
A complete list can be found at www.iana.org/assignments/port-numbers
ARP
Computers, or other hardware, that are connected to a TCP/IP–network all have at
least one IP address. The IP address is also known as the logical address as it is usually
implemented in software and can be changed depending on where in the network the
hardware is physically located. The devices also have a physical address which in an
Ethernet network is called the MAC-address, this is unique for each piece of connect-
ed hardware.
When two pieces of equipment (A) and (B) utilises TCP/IP to communicate over
Ethernet, they must keep track of each other’s MAC-address, as all communication on
an Ethernet is made to MAC-addresses.
A
B
C
LAN
Sub net
B A C K