Welcome to part two of our discussion on routers in our Networking Fundamentals series. In this lesson, we will build upon the concepts covered in part one and delve deeper into how routers forward packets and maintain communication across networks. If you haven’t seen part one yet, we recommend you start there before continuing with this video.
Contents
Understanding Routing Tables and ARP Tables
Routers play a crucial role in directing network traffic by utilizing routing tables and ARP tables. Routing tables act as a map of networks known to a router and help determine the best path for forwarding packets. On the other hand, ARP (Address Resolution Protocol) tables maintain a mapping of IP addresses to MAC addresses for all the nodes in directly connected networks.
The Address Resolution Process
When a router receives a packet with an unknown destination, it consults its routing table to determine the next hop. If the router doesn’t have the destination’s MAC address in its ARP table, it initiates the address resolution process (ARP) to resolve it.
Forwarding Packets from Host A to Host C
Let’s illustrate how routers use their routing and ARP tables to forward packets between hosts. Imagine Host A wants to send data to Host C. Host A constructs a layer 3 header with its source and destination IP addresses.
Since Host A knows that the destination IP address is on a different network, it sends the packet to its default gateway, which is Router 1. However, Host A doesn’t know Router 1’s MAC address. So, it sends an ARP request to learn this information.
Router 1 receives the ARP request and adds Host A’s MAC address to its ARP table. It then responds with its own ARP entry, allowing Host A to populate its ARP table. With the necessary information, Host A can construct a layer 2 header and send the packet to Router 1.
Router 1 receives the packet, discards the layer 2 header, and consults its routing table. It determines that the next hop for this packet is Router 2. However, Router 1 doesn’t have Router 2’s MAC address, so it initiates the address resolution process.
Router 2 receives the ARP request from Router 1, adds the sender’s MAC address to its ARP table, and responds with the necessary ARP entry. Now equipped with the MAC address, Router 1 can construct a layer 2 header and forward the packet to Router 2.
Router 2 receives the packet, discards the layer 2 header, and consults its routing table. It determines that the packet needs to be delivered out of the left interface, as it is the final hop in this scenario. With the MAC address for Host C missing, Router 2 initiates the address resolution process.
Host C receives the ARP request from Router 2, adds Router 2’s MAC address to its ARP table, and responds with the appropriate mapping. Router 2 populates its ARP table and creates a layer 2 header to forward the packet to Host C.
Finally, Host C receives the packet, discards the layer 2 header, and processes the data.
FAQs
Q: How do routers determine the best path for forwarding packets?
A: Routers use their routing tables, which contain a map of networks. By matching the destination IP address of a packet with the entries in the routing table, routers can determine the next hop for the packet.
Q: What is the purpose of ARP tables in routers?
A: ARP tables maintain a mapping of IP addresses to MAC addresses for the nodes in directly connected networks. This information is crucial for routers to construct layer 2 headers and forward packets to the correct destination.
Q: What happens if a router doesn’t have the MAC address of the next hop?
A: The router initiates the address resolution process (ARP) to discover the MAC address. It sends an ARP request to the network, and the device with the matching IP address responds with its MAC address, allowing the router to complete the layer 2 header.
Conclusion
Routers play a vital role in the smooth flow of data across networks. By utilizing routing tables and ARP tables, routers can determine the best path for forwarding packets and maintain communication between hosts. Understanding how routers operate is essential for anyone working with computer networks.
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