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The router 89
of the network. On the other hand, the network interface also has a MAC address on the LAN
side. The router will function as default gateway on the network.
Figure 6.3: Linking of a private network to the Internet via a router
A network can be linked to the Internet via the WAN interface of the router. For this purpose,
the router, usually via DHCP, gets a unique IP address on the Internet from the ISP (Internet
Service Provider).
Every device on the network can now be configured as follows.
IP address 172.23.22.14
Subnetmask 255.255.0.0
Default gateway 172.23.0.1
Thus every participant gets an IP address where the Net ID for every participant is the same
but the Host ID for every participant is unique.
If an application on a PC on the network wants to start a communication with a Server on
the Internet, then the PC first has to build a first IP packet to request the connection. This IP
packet is sent on the Internet via the default gateway. The PC encapsulates the IP packet in
an Ethernet frame for this purpose. Figure 6.4 shows the data that are required to make the
Ethernet frame. The MAC address of the router is obtained via the ARP protocol.
Once the message has arrived at the router, this will forward the IP packet via the WAN
interface to another router on the Internet. As the private network is separated from the
Internet, the router will replace the source IP address of the PC with its IP address on the
WAN side. The private network can only be reached via this external router IP address, via
the Internet.
The server can now send a reply and will address this reply to the external IP address of the
router. A problem occurs now as the router will have to decide to which PC this message
has to be sent. The Server’s response does not contain any data anymore with regard to the
original sender IP NAT was developed to solve this problem.