---
title: "Significance and operation of router"
subject: "Computer Networks and Security"
chapter: 4
key: "semester_7/computer_networks_and_security:router-significance"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/router-significance
site: BCT Engineering
---

# Significance and operation of router

Computer Networks and Security · Chapter 4: Network Layer

- Asked in: 2070 Asar, 4 marks
- Video: [Lec-15: Routers in Computer Networks | Physical, data link and network layer device](https://www.youtube.com/watch?v=JhBnOamc_8s) — Gate Smashers
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/router-significance
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-4

## Question

Discuss the significance of router in computer networking along with its operation.

## Short answer

Routers connect networks, forward packets via IP addresses, select optimal paths, separate broadcast domains, provide security/NAT, and interconnect diverse link technologies. Operation: receive frame, verify CRC, strip L2 header, lookup destination IP via longest prefix match, decrement TTL, recalc checksum, resolve next-hop MAC via ARP, encapsulate in new frame, transmit.

## Answer

A **router** is a layer-3 device that connects different networks and forwards packets between them using the destination IP address.

#### Significance
- Connects LANs to each other and to the Internet / WAN.
- Selects the best path using routing tables and protocols.
- Separates broadcast domains, so broadcasts do not flood the entire network.
- Provides security and control: ACLs, NAT, firewall features.
- Joins different link technologies (Ethernet to serial/DSL) and fragments packets if needed.
- Provides DHCP, QoS and load balancing in practice.

#### Operation (forwarding a packet)
1. The router receives a frame on an interface, checks the frame (CRC) and removes the layer-2 header.
2. It reads the destination IP address from the IP header and verifies the header checksum.
3. It looks up the routing table using the **longest prefix match**; if nothing matches, it uses the default route or drops the packet.
4. It decrements TTL (if 0, drops and sends ICMP time exceeded) and recalculates the checksum.
5. It finds the next-hop MAC address (ARP table) and builds a new frame for the outgoing link.
6. It queues the packet and transmits it on the outgoing interface.

```text
LAN A --[ R1 ]-- WAN --[ R2 ]-- LAN B
        table:           table:
      B -> via R2      B -> direct
```

Meanwhile, a routing protocol (RIP/OSPF) updates the routing table with neighbouring routers.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. On which basis does a router forward packets between networks?
   A. Destination MAC address in the frame
   B. Source port number in the TCP header
   C. Destination IP address in the IP header
   D. VLAN tag of the frame
   Answer: C. A router is a layer-3 device and forwards using the destination IP address. (Key point: Layer and forwarding basis)

2. How does a router help control broadcast traffic?
   A. It encrypts broadcast frames
   B. It separates broadcast domains so broadcasts do not flood the whole network
   C. It converts broadcasts to unicast on every port
   D. It forwards every broadcast faster than a switch
   Answer: B. Routers do not forward broadcasts, so each interface forms its own broadcast domain. (Key point: Broadcast domains)

3. When a router looks up the routing table for a destination, which entry does it choose?
   A. The entry with the longest matching prefix
   B. The first entry in the table
   C. The entry with the shortest prefix
   D. The entry with the lowest MAC address
   Answer: A. The longest prefix match gives the most specific route; if none matches, the default route is used or the packet is dropped. (Key point: Longest prefix match)

4. What does a router do when the TTL of a packet reaches 0 after decrementing?
   A. Forwards it with TTL reset to 64
   B. Sends it back to the sender unchanged
   C. Holds it until the routing table updates
   D. Drops it and sends an ICMP time exceeded message
   Answer: D. TTL = 0 means the packet has used its hop limit, so the router discards it and reports ICMP time exceeded. (Key point: TTL handling)

## Related in Computer Networks and Security

- [Repeater, hub, bridge, switch, router](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Anetwork-devices-working) — Computer Networks and Security, chapter 3
- [Significance of OSI layers](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aosi-significance-layers) — Computer Networks and Security, chapter 1
- [Media Access Control and significance](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Amac-significance) — Computer Networks and Security, chapter 3
- [Data Encryption Standard operation](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Ades-algorithm) — Computer Networks and Security, chapter 8

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