ELSA Documentation


IP-routing-table

The routing table can contain a maximum of 128 entries of destination network addresses or direct IP addresses with netmasks, and the names or IP addresses of other local routers. Alternatively, you can enter a setting by means of which packets to specific destination IP addresses are discarded and are not answered by proxy ARP. This is done by entering 0.0.0.0 for the name of the responsible router.

The 'Masquerade' field indicates whether the route should be masked or not. The following options are offered here:

O On: IP masquerading is switched on and functions with dynamic assignment of the IP address by the remote station. In this procedure the router queries the IP address '0.0.0.0' at the remote station and is assigned a random IP address by the remote station, which is then used for further processing.
O Off: Masquerading is switched off.
O Static: IP masquerading is switched on and functions with assignment of a static IP address previously assigned by the remote station. In this procedure the router queries the IP address entered under 'Setup/TCP-IP-module' at the remote station and is assigned this address by the remote station. Use this setting when the remote station (e.g. your Internet provider) has assigned you a fixed IP address in the access data. Of course, this procedure will only function when this address has also been entered in the router as the IP address.

The IP routing table is generally sorted as shown below:

O The longest network mask is placed on top.
O For network masks of equal length, the one with the smallest IP address is placed on top.

In order to identify the correct remote station, the router searches the routing table from top to bottom using the destination IP address received. If a matching entry is found, the router name found is used for establishing the connection.

Address ranges that are prohibited in the Internet are excluded from transmission by preset entries in the IP routing table (the router name 0.0.0.0 means that packets to these addresses are not transmitted). The IP routing table below is provided by way of example and also shows the default settings:

IP-address IP-netmask Router-name Distance Masquerade
192.168.0.0 255.255.0.0 0.0.0.0 0 Off
172.16.0.0 255.240.0.0 0.0.0.0 0 Off
10.0.0.0 255.0.0.0 0.0.0.0 0 Off
224.0.0.0 224.0.0.0 0.0.0.0 0 Off

However, if these addresses are required for Intranet use, for example, it is possible to delete the predefined entries at any time. If the routing table contains no entries with the router name 0.0.0.0, the router processes all IP addresses with valid routes.

O Example
o The local network address is 192.120.130.0.
o Three terminal units must be available via proxy ARP with the IP addresses 192.120.130.10, 192.120.130.11 and 192.120.130.12 via an ELSA LANCOM 'Leeds'.
o Two destination networks 192.120.131.0 and 192.120.132.0 can be accessed by the remote stations 'GLASGOW' and 'LONDON'.
o Data packets for the destination network 193.140.300.0 are to be sent to another local router with the IP address 192.120.130.200.
o Absolutely nothing is to be transmitted to the destination network 193.140.200.0.
o All other non-local data packets must be sent to the router 'PROVIDER' at the Internet service provider.
  In this example, the router table would contain the following entries:

IP-address IP-netmask Router-name Distance Masq.
192.120.130.10 255.255.255.255 LEEDS 0 Off
192.120.130.11 255.255.255.255 LEEDS 0 Off
192.120.130.12 255.255.255.255 LEEDS 0 Off
192.120.131.0 255.255.255.0 GLASGOW 0 Off
192.120.132.0 255.255.255.0 LONDON 0 Off
193.140.200.0 255.255.255.0 0.0.0.0 0 Off
193.140.300.0 255.255.255.0 192.120.130.200 0 Off
255.255.255.255 0.0.0.0 Service provider 0 On

NOTE: If the connection to the selected remote station is to be realized via a PPP connection, the IP rights must be enabled for the corresponding entry in the PPP table.

The last line is an entry for the "default route". The IP address 255.255.255.255 means the same as 0.0.0.0 (for technical reasons, 0.0.0.0 cannot be entered in the first column). Because it contains the IP network mask 0.0.0.0, this line is always appropriate after the rest of the table has been searched. Therefore, the router sends everything that it cannot transfer over other routes and should not discard or that comes from a WAN terminal and is not local to the router at the provider.

To route all data packets to a specific network to another LAN, e.g. via an ISDN router in the local network, enter the IP address of the other network and the netmask into the table and specify the local IP address of the ISDN router under 'Router'. For example, you are using IP addresses from the 10.1.0.0 (netmask 255.255.255.0) address range in your local network. The ISDN router has the local IP address '10.1.0.99'; the other local network (your branch office) uses 10.2.0.0 (netmask 255.255.255.0). The following entries will ensure that the cable modem sends all data packets for the other network to the ISDN router and the remaining data packets to the cable network (insofar as they are not in the prohibited ranges):

IP-address IP-netmask Router-name Distance
192.168.0.0 255.255.0.0 0.0.0.0 0
172.16.0.0 255.240.0.0 0.0.0.0 0
10.2.0.0 255.255.255.0 10.1.0.99 0
10.0.0.0 255.0.0.0 0.0.0.0 0
255.255.255.255 0.0.0.0 Cable 0

NOTE: Also enable local routing for this function!


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