A router's two functions are: (1) determine the best path to the destination using its routing table, and (2) forward the packet toward that destination by encapsulating it in the appropriate Layer 2 frame.

* A router's two jobs: path determination and forwarding. *

Function 1 — Path Determination:

• Router receives a packet on an interface
• Examines the destination IP (Internet Protocol) address
• Searches its routing table for the best matching route
• The best match is determined by the longest prefix match — the route with the most specific (longest) subnet mask wins

Function 2 — Packet Forwarding:

• After determining the best path, the router identifies the exit interface and next-hop IP
• De-encapsulates the received Layer 2 frame
• Re-encapsulates the packet in a new Layer 2 frame for the exit interface
• Forwards the new frame out the exit interface

Key insight: The IP (Internet Protocol) packet (Layer 3) stays essentially unchanged hop-by-hop — only the TTL (Time to Live) decrements. But the Layer 2 frame (Ethernet header) is rebuilt at every router hop with new source and destination MACs (Media Access Control addresses).

Tip: Think of the router as a mail sorting facility: it reads the destination address (IP), determines which truck goes to that zip code (routing table), and puts the letter in a new envelope for the next leg of the journey (new L2 frame).

Go deeper:

• Router (computing) (Wikipedia) — the path-determination plus forwarding split the card describes.
• Routing (Wikipedia) — separates route selection from per-packet forwarding.

The show ip route output displays all known routes with their source codes, destination networks, prefix lengths, next-hop addresses, and exit interfaces. The router searches this table for each packet's destination IP (Internet Protocol) using longest prefix match.

Reading the routing table output:

R1# show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

Gateway of Last Resort is not set

C    192.168.1.0/24 is directly connected, GigabitEthernet0/0
L    192.168.1.1/32 is directly connected, GigabitEthernet0/0

Route codes and what they mean:

Tip: Routers use the routing table like a map to find the best path for a given destination. The more specific the route (longer prefix), the more preferred it is.

Go deeper:

• Routing table (Wikipedia) — the fields (destination/netmask, next hop, interface, metric) the output renders.