HCNA Networking Study Guide by Huawei Technologies Co. Ltd

Author:Huawei Technologies Co., Ltd.
Language: eng
Format: epub
Publisher: Springer Singapore, Singapore

Destination MAC address: MAC address of Interface 1 on Router A

Unicast frame sent by Router A

Source MAC address: MAC address of Interface 2 on Router A

Destination MAC address: MAC address of Interface 1 on Router B

Unicast frame sent by Router B

Source MAC address: MAC address of Interface 2 on Router B

Destination MAC address: MAC address of network interface on PC 2

This indicates that the frame received by PC 2 is not the frame sent by PC 1—PC 1 and PC 2 cannot exchange frames with one another. Layer 2 (data link layer) communication between PC 1 and PC 2 is blocked by the router. However, the IP packet received by the network layer of PC 2 is the original IP packet sent by the network layer of PC 1, so these two PCs are able to exchange IP packets. In other words, Layer 3 (network layer) communication is implemented between PC 1 and PC 2.

Routers and internets

The development of routers led to the development of internets, or interconnected networks (note that our use of the word “internet” here is deliberately spelled with a lower-case letter I, not to be confused with “the Internet”). Figure 6.5 shows the same network in Fig. 6.4, reconfigured to illustrate the concept of an internet.

Fig. 6.5Example of an internet

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