Module 14: Routing Between Networks

The Need for Routing

As networks expand, they are often divided into multiple access layer networks for efficiency, security, and scalability. Routers play a crucial role in managing traffic between these networks. Networks can be divided based on several factors:

1. Broadcast Containment – Routers at the distribution layer limit broadcast traffic to the local network where it is needed.
2. Security Requirements – Sensitive data can be isolated within specific subnets to protect confidential information.
3. Physical Locations – Routers interconnect networks at different geographical locations within an organization.
4. Logical Grouping – Networks can be structured based on logical groups, such as company departments with shared resources.

The distribution layer ensures that internal network traffic remains local while enabling communication between different subnets as needed.

Router Functionality

A router is a Layer 3 networking device responsible for connecting multiple IP networks and directing traffic efficiently. Unlike switches that make decisions based on MAC addresses (Layer 2), routers forward packets based on IP addresses (Layer 3). Whenever a source and destination device have different network portions in their IP addresses, a router is required to forward the message.

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The Routing Table

Each router interface connects to a different local network. A router maintains a routing table that contains information about directly connected networks and the best available paths to reach other networks.

Routing Process:

1. The router receives an incoming frame and extracts the destination IP address from the packet.
2. It matches the network portion of the destination IP address with the entries in the routing table.
3. If a match is found, the router encapsulates the packet in a new frame and forwards it through the appropriate interface.
4. The router either delivers the packet to the destination network or forwards it to another router along the path.

Router and MAC Address Resolution:

• When forwarding a packet to another router, the sending router needs the MAC address of the next-hop router.
• The router obtains this MAC address from its ARP table.
• Hosts rely on the default gateway address (the IP of the router’s interface) to send packets outside their local network.

Routing Table Entries:

Routing table entries can be created in two ways:

• Dynamic Routing – Automatically updated using information exchanged between routers.
• Static Routing – Manually configured by a network administrator.

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Creating a LAN

A Local Area Network (LAN) consists of interconnected networks under the same administrative control. Common LAN characteristics include:

• Use of Ethernet or wireless protocols.
• Support for high data transfer rates.
• Typically managed within a single organization.

Single vs. Multiple Networks in a LAN:

• Single Local Network:
  • All hosts are within the same broadcast domain.
  • Hosts communicate using ARP to resolve MAC addresses.
  • Simpler network design, but increased broadcast traffic.
• Multiple Local Networks:
  • Traffic is divided across multiple subnets.
  • Reduces broadcast-related congestion.
  • Requires routing for inter-network communication.

Routing Considerations in a LAN:

• Routers are required for hosts on different subnets to communicate.
• Routing increases network complexity and introduces potential latency.
• Efficient routing and network segmentation improve overall network performance.

By implementing routing strategies, network administrators can optimize traffic flow, enhance security, and ensure efficient communication between devices across multiple networks.