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Discovering computer networks: hands on in the Open Networking Lab
Discovering computer networks: hands on in the Open Networking Lab

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2.3 Networks of networks

In this part you will look at the internet-facing side of the home gateway. In particular, you’ll look at its IP address and subnet mask, which are very different from those on the home gateway side. This leads to an exploration of what is on the ‘other side’ of the gateway. For shorthand, this is usually referred to simply as ‘the internet’, but the internet is actually a vast network of networks. Networks are linked to other networks by routers. The router’s job is to transfer data packets from one network to another, according to the packet’s destination IP address. The important concepts of latency and hopping are introduced.

Now watch the video below, which is about 6 minutes long.

Networks of networks

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Now try to answer the questions below.

Activity 3 Test yourself

10 minutes

  • 1. What aspect of data transmission using the Internet Protocol is likely to cause problems for ‘real-time’ exchange such as a web conference or video call?

  • The latency of the exchange can cause awkward gaps when neither side knows whether the other side is speaking. Also, the lack of guaranteed delivery could be a problem (although the loss of the odd packet is not disastrous for a conversation).

  • 2. The packets of a data transmission do not necessarily arrive at the destination in the right order. Why?

  • Packets can arrive out of sequence because they do not necessarily all take the same route. Some routes might involve more hops than others. More hops mean more routers visited, and routers introduce delay. An early packet could take a longer route than a later packet, and therefore arrive after it.

  • 3. If packets arrive out of sequence, they are assembled into the correct sequence by a protocol known as Transmission Control Protocol (TCP) at the destination computer. Why would it not be a good idea to use TCP for a ‘real-time’ exchange?

  • Using TCP would increase the delay (or latency). It would take time to assemble packets in the right order, and TCP might wait too long a time for a packet that never arrives.

  • 4. The video shows an IP address of 99.0.0.1 being used for the gateway of the internet service provider (ISP). A subnet mask of 255.0.0.0 is used with this address. Comment on the number of devices this network could support, and whether there would be a problem using the kind of fault-finding approach used in Section 2.2.

  • Potentially there could be very many devices. With a 255.255.255.0 subnet mask, only the last number of the IP address is available for devices. With 255.0.0.0, the last three numbers of the IP address are available for devices. This would make fault-finding very difficult if every device had to be individually investigated.