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Welcome to the course devoted to sudden netting and its bearing on the security of the whole computer

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infrastructure

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let's start from the very beginning.

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For to host to be able to communicate in computer networks IP networks.

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They must have unique IP addresses.

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If two hosts are in the same network that can communicate with each other directly if two hosts are

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located in remote networks all communication between them takes place through routers devices that are

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responsible for connecting individual networks.

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This means that by dividing the company into smaller subnets you gain the ability to control the packets

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transmitted between hosts.

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The first advantage of this solution is that the packets will always be sent by rodders that you manage

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yourself.

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You don't need to use routers when you divide the network into subnets.

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Instead you'll use a special kind of routers namely firewalls the router forward's packets.

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It must reject erroneous packets or those that exceeded their lifetime firewalls on the other hand are

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routers that filter forward data this enables you to improve not only the efficiency but also the security

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in a direct way.

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If you remember lecture's devoted to the consumerization of I.T. you will know that our corporate network

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encompasses more than just devices in the company's headquarters

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at present a corporate computer network can include users temporary whereabouts.

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These are very often users homes because they communicate with the network through mobile devices.

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Such a network can hardly be regarded as trusted.

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Not all administrators know what devices are connected to their networks.

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When a local corporate network included six hosts and two servers only the distinction between the secure

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corporate network and a dangerous Internet was reasonable.

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Right now our corporate network encompasses every single device that any given user connects to it.

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Therefore the security level of such a network is similar to the one of an external network it needs

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to be divided into smaller segments.

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It will allow us to monitor packets sent over a network and in particular to control the availability

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of selected hosts.

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If you use firewalls to create subnets and computers and subnet will be available only for trusted hosts

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located in subnets B and C

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using firewalls instead of routers you gain the ability to detect and prevent attacks such as those

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that you see in the picture below.

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What we see there is a network communication between two clients first intercepted and then reassembled.

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If we reassemble such communication that is what data was sent in the TCAP connection it will turn out

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that it was a computer trying to connect to a web server in order to download a file root see.

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You wouldn't like your computer to download and run such a file the application layer firewalls offer

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the ability to monitor the network these firewalls employ more than just simple rules.

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Such as a computer from network can communicate with the computer from Network B on port 80 there are

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firewalls that examined only protocol headers but also data transferred by these protocols.

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Let's focus again on the IP protocol on its fourth version.

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The simplest method of sudden netting is classful addressing as you can see in the picture above.

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Three classes are used a B and C Class D is used for multicasting addresses are reserved for future

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use classful addressing is very simple to distinguish a network address from a host address.

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You look at the first bits of the most significant byte This is the first part of the IP version 4 address.

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If the first bit of the octet is 0 then this is a Class A address in decimal notation.

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These would be values ranging from 0 to 126 the value 127 is the loopback and any address starting with

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one to 7 points out to your local computer the same method applies to higher classes.

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If the address belongs to class B the value of the oldest bit must be 1 and the next one minus zero.

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This means that the first two octets constitute the network address and the next to the host address.

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If the first two bits of the last octet are set to 1 and the third one is set to zero then the IP address

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belongs to class C in such a situation the first three octets constitute network address and the fourth

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one is the host address.

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It's a clear and simple principle.

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Unfortunately such address thing is very inefficient.

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Many addresses are wasted in the case of IP version 4 protocol.

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You cannot purchase just any address if you want to achieve proper routing.

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You always have to have a network address even if your network includes six computers.

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You still have to buy a whole class C network address.

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You have to pay for all of the 256 addresses even though you'll use only 8 of them.

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The solution to this problem is classless in her domain routing.

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See your utilizes the fact that the network address and the host address don't need to be divided into

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individual octets one part of an actor can define the network address and the other the host address.

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To allocate these bits a CD-R mask is needed.

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I'm sure you remember from the previous lectures to see IDR addressing scheme the complete network address

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must come before the host address.

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The only correct masks are shown in the table below.

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Zeros and ones cannot appear interchangeably in a mask.

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Ones indicate the network address and zeros indicate the host address.