Gateway; Ip Packet; Tcp Protocol; Tcp Port Numbers - WAGO -I/O-SYSTEM 750 User's Installation And Configuration

276 • ETHERNET
Network communication

Gateway

The subnets of the Internet are normally connected via gateways. The function
of these gateways is to forward packets to other networks or subnets.
This means that in addition to the IP address and network mask for each net-
work card it is necessary to specify the correct IP address of the standard gate-
way for a PC or fieldbus node connected to the Internet. You should also be
able to obtain this IP address from your network administrator.
The IP function is limited to the local subnet if this address is not specified.

IP Packet

In addition to the data units to be transported, the IP data packets contain a
range of address information and additional information in the packet header.
Fig. 5-11: IP packet
The most important information in the IP header is the IP address of the
transmitter and the receiver and the transport protocol used.
5.2.2.3

TCP protocol

As the layer above the Internet protocol, TCP (Transmission Control Proto-
col), guarantees the secure transport of data through the network.
TCP enables two subscribers to establish a connection for the duration of the
data transmission. Communication takes place in full-duplex mode, i.e. trans-
mission between two subscribers in both directions simultaneously.
TCP provides the transmitted message with a 16-bit checksum and each data
packet with a sequence number.
The receiver checks that the packet has been correctly received on the basis of
the checksum and then sets off the sequence number. The result is known as
the acknowledgement number and is returned with the next self-sent packet as
an acknowledgement.
This ensures that the lost TCP packets are detected and resent, if necessary, in
the correct sequence.

TCP port numbers

TCP can, in addition to the IP address (network and subscriber address), re-
spond to a specific application (service) on the addressed subscriber. For this
the applications located on a subscriber, such as i.e. web server, FTP server
and others are addressed via different port numbers. Well-known applications
are assigned fixed ports to which each application can refer when a connection
is built up.
Examples:
A complete list of "standardized services" is contained in the RFC 1700 (1994)
specifications.
IP-Header IP-Data
Telnet Port number: 23
HTTP Port number: 80
Modular I/O System
ETHERNET TCP/IP