Netopia R3100-U User Reference Manual page 265

Routers B and C (which could also be Netopia R3100s) serve the two remote networks that are subnets of
a.b.c.0. The subnetting is accomplished by configuring the Netopia R3100 with connection profiles for Routers
B and C (see the following table).
Connection profile
for Router B
for Router C
The Netopia R3100's connection profiles for Routers B and C create entries in its IP routing table. One entry
points to the subnet a.b.c.128, while a second entry points to the subnet a.b.c.248. The IP routing table might
look similar to the following:
Network Address-Subnet Mask-----via Router------Port--Age--------Type------
----------------------------------SCROLL UP--------------------------------
0.0.0.0
127.0.0.1
a.b.c.128
a.b.c.248
---------------------------------SCROLL DOWN-------------------------------
UPDATE
Let's see how a packet from the Internet gets routed to the host with IP address a.b.c.249, which is served by
Router C. The packet first arrives at Router A, which delivers it to its local network (a.b.c.0). The packet is then
received by the Netopia R3100, which examines its destination IP address.
The Netopia R3100 compares the packet's destination IP address with the routes in its IP routing table. It
begins with the route at the bottom of the list and works up until there's a match or the route to the default
gateway is reached.
When a.b.c.249 is masked by the first route's subnet mask, it yields a.b.c.248, which matches the network
address in the route. The Netopia R3100 uses the connection profile associated with the route to connect to
Router C, and then forwards the packet. Router C delivers the packet to the host on its local network.
Remote IP address
a.b.c.128
a.b.c.248
0.0.0.0
255.255.255.255 127.0.0.1 lp1
255.255.255.192 a.b.c.128 WAN
255.255.255.248 a.b.c.248 WAN
Remote IP mask
255.255.255.192
255.255.255.248
IP Routing Table
a.b.c.1 WAN
Understanding IP Addressing E-11
Bits available for host
address
7
3
3719 Management
6423 Local
5157 Local
6205 Local