Remember that the LAN interfaces on routers, such as an Ethernet
interface, perform both the layer 2 function of a switch and the layer
3 function of a router. Like switches, routers segment each LAN
interface into a separate collision domain as shown in Figure .
However, routers separate LAN and WAN segments into different networks
or subnetworks (layer 3). This means that routers not only separate
interfaces into their own segments, they also do not propagate
(forward) layer 2 broadcast requests, such as ARP Requests, out other
interfaces.
Routers interconnect different networks or subnetworks. In Figure
,
the router is connecting two different subnetworks, 172.30.1.0/24 and
172.30.2.0/24. The router has an Ethernet interface on each subnetwork,
Ethernet 0 with the IP address 172.30.1.1/24 and Ethernet 1 with the
IP address 172.30.2.1/24. The router will only forward packets from
one subnetwork to the other if the destination IP address is on the
other subnetwork. Because, the destination IP address is a layer 3
address, as opposed to the layer 2 addresses, the router will not
forward layer 2 broadcast addresses like ARP Requests. Therefore, the
router separates or segments, the network into separate broadcast
domains.
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