ELSA Cable manual Expansion through local networks

Technical basics 57

Media access control

LAN and IP network

All the router has to remember is that every address beginning with '192.168.100' is located within the network of Example Inc.

Now imagine a router that is connected to the network of Example Inc. through an interface. If it receives a packet with destination address '192.168.100.4' and netmask '255.255.255.0', it will compare this with every network address it knows. In doing so it carries out a logical AND with the netmask, and compares the results with the network address: '192.168.100.4' AND '255.255.255.0' is '192.168.100.0'. This is the network address of the Example Inc. network. The router recognizes that the recipient is located within Example Inc. and passes the packet on to the appropriate interface for Example Inc. Within Example Inc. the packet is then passed on to the appropriate subnet.

The same procedure is used for the transfer of IP packets within a network:

aIf a host in the subnet of the development department wants to send a data packet to Mr. 'Smith', the sender attaches the destination address "Host 'Smith' – Marketing – Example Inc.".

bThe router in the development division receives the packet and extracts from the address the information that it is directed at the marketing division of Example Inc. Since it is itself part of Example Inc., but not of the marketing division, it passes the packet on to the router in the superordinate network.

cThe router at the Example Inc. receives the packet and extracts from the address the information that it is directed at Example Inc. Since it is itself part of Example Inc., it takes a closer look at the address to find the name of the division. It then passes the packet on to the router in the marketing division, where the packet is passed on to the recipient.

Expansion through local networks

Up to now we have only considered the point-to-point connections. However, many computer networks are based on multipoint cabling such as Ethernet. All computers connected to the same network can then receive the signals of all other computers (so- called broadcast transfer to a shared medium). If several computers are sending simultaneously, the superimposed signals are destroyed. A variety of access methods such as CSMA/CD or Token Ring are implemented in the MAC layer (Media Access Control, MAC) for the avoidance and resolution of such collisions.

The connection of all computers communicating through a shared medium using a MAC protocol is called a LAN. A LAN forms an independent network and is subordinate to the IP network, i.e. IP networks can use the physical connections of the LAN to establish connections between the hosts and the routers. LAN refers to a limitation of the area covered by the network, not a restriction of the number of workstations connected to it.

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