- Routers (or gateways) are special purpose machines on the Internet that determine the path for packets from source to destination
- when a router receives a packet, inspects the destination address
- looks up that address in a routing table
- based on the contents of the table, forwards the packet to another router (or to its final destination if possible)
- Routing Information Protocol (RIP)
- describes how routers exchange routing table information
- uses hop-count as the metric of a path's cost
- Open Shortest Path First Protocol (OSPF)
- more robust, scalable protocol than RIP
- doesn't exchange entire tables, only updates changed links
- Internet Control Message Protocol (ICMP)
- adjunct to IP, notifies sender (or other router) of abnormal events
- e.g., unreachable host, net congestion
