Introduction

Have you ever wondered how an email sent from your device reaches its intended recipient, especially when they are outside your local network? This seemingly simple process involves multiple layers of the OSI model and several addressing schemes working in tandem to ensure accurate and efficient data transmission. In this blog post, we will explore the roles of source and destination addresses at different layers of the OSI model when sending an email outside the local network.

Creating and Sending an Email

When you compose and send an email, your email client creates a message with the necessary headers, such as the sender’s and recipient’s email addresses, subject, and message content. The email client then connects to the Simple Mail Transfer Protocol (SMTP) server responsible for sending the email. To route the email to the recipient’s mail server, your email client communicates with the Domain Name System (DNS) to resolve the recipient’s email domain to the IP address of their mail server.

Layer 2 and Layer 3 Addresses

Throughout this process, both Layer 2 (Data Link Layer) and Layer 3 (Network Layer) addresses are used to transmit the data across the network. Here’s a brief overview of what happens with source and destination addresses at each layer:

Layer 2 (Data Link Layer):

• Source MAC Address: Your device’s Network Interface Card (NIC) MAC address serves as the source MAC address.
• Destination MAC Address: Initially, the destination MAC address will be that of the gateway or router responsible for forwarding the data outside your local network. As the data passes through multiple routers, the destination MAC address changes to represent the next hop in the path towards the final destination.

Layer 3 (Network Layer):

• Source IP Address: Your device’s IP address serves as the source IP address. This address remains unchanged throughout the entire process.
• Destination IP Address: The destination IP address is the IP address of the recipient’s mail server. This address remains unchanged throughout the entire process.

The Role of Routers

As the email moves through the network, routers play a critical role in examining the destination IP address to determine the best path to forward the packet. Each router in the path updates the Layer 2 destination MAC address to ensure the packet reaches the next hop. Finally, the email arrives at the recipient’s mail server, which processes the message and makes it available for the recipient to access via their email client.

Conclusion

Understanding the roles of source and destination addresses at different layers of the OSI model when sending an email outside the local network provides valuable insights into how data is transmitted across networks. This knowledge helps network professionals comprehend the intricacies of network communication, develop strategies to optimize network performance, and troubleshoot potential issues. Keep learning and expanding your networking expertise with our informative posts on Network ThinkTank!