ELSA Cable manual IP masquerading NAT, PAT, Two addresses for the router

36Operating modes and functions

Module/Local Routing On menu). This tells the router in your device to send the data packet to the other responsible router. The router will then no longer send any ICMP redirects.

This may seem to be a good idea in principle, but local routing should still only be used as a last resort, since this function leads to doubling of the number of data packets being sent to the destination network required. The data is first sent to the default router and is then sent on from there to the router in the local net which is actually responsible.

IP masquerading (NAT, PAT)

One continually growing problem for the Internet is the limited number of generally valid IP addresses available. In addition to this, the allocation of fixed IP addresses for the Internet by the Network Information Center (NIC) is an expensive process. What is more obvious than having several computers share one IP address?

This particular solution is called IP masquerading. This is a procedure whereby only one LAN router appears on the Internet with an IP address. This IP address is allocated to the router either permanently by the NIC or temporarily by an Internet provider. All the other computers on the network then “conceal“ themselves behind this one IP address. Aside from the welcome savings, IP masquerading has the added benefit of guarding very effectively against attacks on the local network from the Internet.

The IP masquerading function is connected to the operating mode of the cable modem as a router. Whenever routers are mentioned in the following paragraphs, this is a reference to the cable modem in the operating mode of an IP router.

Two addresses for the router

Masquerading pits two opposing requirements of the router against one another:

KOn the one hand, it has to have a valid IP address in the local network of the user so that it can be reached from the LAN.

KOn the other hand, it has to have a valid address in the cable network.

Since these two addresses may not in principle be located on the same logical network, there is only one solution: two IP addresses are required.

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