Datasheet
INSYS GPRS 5.0 Ethernet Functions
Dez-06 33
The figure shows two private network segments, which do not know about each other,
but both need access to the web server www.insys-tec.de.
Both private network segments use the same IP addresses, which is possible for private
IP addresses as they are not routed in the public Internet. As non of the LAN devices from
the private network segments are visible in the public Internet (due to their non-routable
IP addresses), they will need a proxy.
This proxy has two IP interfaces. One interface is used for the local LAN (Local Area Net-
work) and the other one is used for the public WAN (Wide Area Network).
On the WAN-side, the proxy will receive a publicly accessible IP address. On its LAN-side,
it will receive an IP address from the private address range of the according network
segment.
The INSYS GPRS 5.0 Ethernet represents such a proxy. LAN devices treat it as a router or
gateway, i.e. the LAN IP address of the INSYS GPRS 5.0 Ethernet is entered as gateway for
LAN devices (see 5.3.1 Routing general).
If the INSYS GPRS 5.0 Ethernet receives an IP packet with a destination in the public
Internet, it will - in its function as a proxy - replace the sender IP address by its public IP
address.
At the same time, the INSYS GPRS 5.0 Ethernet will enter
Its own public port number for this connection
The IP address of the local sender computer
The port number of the local sender computer
The used protocol (TCP/UDP)
into a table. This table is called NAT table (Network Address Translation Table) and is the
central module of the INSYS GPRS 5.0 Ethernet.‘
Using the public port number that was entered in the table, it will then send the IP
packet to the destination computer in the public network and will then wait for the re-
sponses of the destination computer at the same port.
For the destination computer, it now seems as if the IP packet was sent by the INSYS
GPRS 5.0 Ethernet, so the destination computer will send the according response IP pack-
ets to exactly this device.
The INSYS GPRS 5.0 Ethernet will now receive its IP packet at this public port and will
forward it to the local computer, which has been entered in the NAT table as local com-
puter with its local port number for this public port.
All entries in the NAT table are dynamic entries, e.g. they are deleted, if one of the follow-
ing conditions occurs:
The sender or the destination computer close the connection by sending an RST
packet
The timeout of the selected protocol (TCP/UDP) has expired, e.g. there was no
data traffic for a certain period of time
The TCP timeout can be defined for the INSYS GPRS 5.0 (see Chapter 5.5.5) The UDP
timeout is 120 seconds.
The NAT table is limited to 1024 simultaneous, possible entries.