|  | 11.7 | ARP Tables
| 11.7.2 | Comparing router ARP tables with ARP tables kept by other networking devices

Routers can be connected to multiple networks, or subnetworks. Generally speaking, network devices map the IP addresses and MAC addresses that they see on a regular and repeated basis. This means that a typical device contains mapping information pertaining only to devices on its own network. It knows very little about devices beyond its LAN.

Routers build tables that describe all networks connected to them. ARP tables kept by routers can contain IP addresses and MAC addresses of devices located on more than one network.

In addition to mapping IP addresses to MAC addresses, router tables also map ports. Can you think of a reason why routers would need to do this? (Note: Examine the router's ARP table below.)

Destination Network | Router Port
--- | ---
201.100.100.0 | 201.100.100.1
201.100.101.0 | 201.100.101.1
201.100.120.0 | 201.100.120.1
201.100.150.0 | 201.100.150.1