Getting into Your Car to Drive to Lunch

Getting into Your Car to Drive to Lunch

When it's time to take a lunch break and your office building isn't close to any restaurants, you might have to get in your car and drive. If you're meeting some friends or business associates for lunch, and you talk before meeting them there, you probably don't bother to mention how you'll be driving to the restaurant. They typically aren't that interested in how you get there.

Likewise, before an IP packet can do the equivalent of driving over the roads, it needs to do the equivalent of getting into a car. Before an IP packet can cross an Ethernet, it has to be encapsulated inside an Ethernet frame. When performing the encapsulation, Hannah happens to overcome two different problems:

The solution to the first problem, as you've probably guessed, is encapsulation. Remember: Each layer in the TCP/IP acrchitecural model provides services to the layer above it. Ethernet sits at the network interface layer of TCP/IP, right below the internetwork layer. To get the packet from Hannah to R1, IP uses Ethernet. Hannah encapsulates the IP packet in an Ethernet frame for transmission over the LAN. Figure 11-3 shows the details.