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Introduction to computer networks

Here you will be introduced to the basic principles of computer networks, how they are built and how they function.
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Hi, my name is John and I’m a senior pirate at Coventry University. Today, I’m going to be introducing the topic of computer networks but before we begin let us consider something else. What is this computer thing anyway? Some
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definitions are: binary machines that store and manipulate information, programmable electronic devices whose function can be adapted by means of loading different programs, overpriced electronic mumbo jumbo, or machines that perform and display the results of logical and mathematical operations. All of these things are true, but they’re not the droids I’m looking for. Computers are also a network of components. Let us have a closer look. This is a mother board. This is where we connect the computer’s CPU, memory and expansion cards etc. Of interest to us are the corridors connecting the parts together. Multiple components like the expansion cards share these corridors and as such they have to take turns to send or receive data.
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The corridors are called ‘buses’ and they require a mechanism to regulate the systems traffic. When we invented the first computer networks we tried to solve the problem by connecting lots of computers together with a wire, meaning that only one computer could send or receive data at any one point in time. Connecting computers in line like this is highly impractical. So we moved to a system like this. At a first glance this looks like an improvement and it does make wiring easier, but it is essentially the same thing. Looking inside a hub, one realises it actually conceals a bus.
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All ports are connected together which means that only one computer can send and receive information at any one point and that every message is broadcast and could be received by anybody on the network. A shared medium means that data can be received and read by everyone and an element of trust has to be involved when using this system. This demonstrates the logic behind the development of early networks. The focus was to create something functional and the security of the system was not the main objective. But the reason that hubs became obsolete had nothing to do with security. We simply wanted bigger networks. And so, we invented switches.
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Switches are one of the most common network devices today and they’re able to connect more systems than a hub as they do not treat every message as a broadcast. Unlike hubs, switches only send the message out of a specific port corresponding to which device. Naturally for this to work we need to know what port corresponds to which device. We can do this by recording the source, media, access control or MAC address of the incoming packets of data in a table. If an entry is logged for the destination then the system will only send the message to that recipient. Otherwise, it could be sent to everyone. This system also involves an element of trust.
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In many modern computers the MAC address assigned by the manufacturer can be changed. One device can impersonate another device. There are also other ways that this mechanism can be exploited. Switching tables can get filled by false advertisements of non-existing addresses or the switch could be forced to create a broadcast by sending messages to non-existing destinations. As we demanded even bigger networks we created more problems to sort out and so we invented routers. Instead of using a MAC address, which is essentially a serial number, a router uses a location-dependent address like an IP address. Those are assigned depending on where we are, thus allowing networks to connect together.
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Typically this is done by broadcasting a request for an IP address and accepting the first offer to arrive and hoping it’s not from a hacker doing a man in the middle attack. On the plus side, routers do finally offer some mechanism for securing our network. They can enforce rules as to where we are allowed to go and what we’re allowed to access. Unfortunately the most common version of internet protocol, IPV4, still has no security features unlike it’s successor-to-be IPV6. Within insecure hardware and low-level protocols, key elements such as encryption, still have to be addressed at the application level. Fundamentally we are trying to patch something that was created without considering the security question.
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We need more than just such a set of rules to secure a network. And even though a bit of healthy paranoia is a major asset for a networking pirate, essentially you can’t build a network around the principle that you do not trust anyone so there will always be problems and vulnerabilities to address.
Here you will be introduced to the basic principles of computer networks, how they are built and how they function.
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Basics of Network Security

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