What does a router do?

A router is a network device that forwards IP packets between different networks and chooses the path those packets will take next.

Short and clear: a router connects different networks and decides where to send a packet next.

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#Main tasks of a router

#1. Connects different networks

A router works when:

• devices are located in different subnets
• data needs to be passed beyond the local network

Without a router, communication between subnets is impossible.

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#2. Analyzes the destination IP address

A router:

• looks at the destination IP address in the packet
• determines which network the recipient belongs to

It does not analyze application data, only the IP header.

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#3. Chooses a route

Using the routing table, a router determines:

• through which interface to send the packet
• to which next hop

If there are several routes, the most suitable one is chosen.

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#4. Forwards the packet further

After choosing a route, a router:

• decreases the TTL value
• wraps the IP packet in a new data link layer frame
• sends it to the next network

This process repeats at every router along the path.

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#5. Separates broadcast domains

A router:

• does not pass broadcast traffic
• limits it to the boundaries of a single subnet

This improves the network's stability and scalability.

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#What a router does NOT do

Important for an interview A router:

• does not forward frames by MAC addresses (that is L2)
• does not guarantee packet delivery
• does not manage TCP connections
• does not analyze application data

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#At what layer a router operates

• main layer: the network layer (L3)
• works with:
  • IP addresses
  • packets
  • routes