Technical basics 59

i.e. all it has to remember is the network address and the netmask of the subnet in the LAN.

IP host

IP host

in the LAN

in the LAN

 

The router in the

 

LAN:

 

knows only the

LAN with router function

address of the LAN

In contrast, the host is confronted with a more difficult task than the router. In case of an interface with a point-to-point cable, the host knows that all packets that it sends through the interface automatically arrive at its router, for example. In case of the point- to-multipoint connection to the LAN, it has to distinguish two cases, however.

KA packet with an address outside the LAN is passed on to a router in the LAN that takes care of the further processing of the packet.

KThe sending host must send a packet with an address within the LAN directly to the target host, since the router in the network does not know the addresses of all the different hosts.

 

Data transfer within the LAN

 

Let's use an example to explain this. Imagine the hosts of the subnet in the marketing

 

division are linked via a LAN. The hosts have IP addresses from the numerical space

 

'137.226.4.1' to '137.226.4.254' (the addresses '137.226.4.0' and '137.226.4.255' are

 

reserved), the network address is '137.226.4.0' and the netmask is '255.255.255.0'. A

 

router connected to the LAN provides access to the wide world of the Internet. Its LAN

 

interface has the IP address '137.226.4.1' and the MAC address '00-80-C7-6D-A4-6E'.

 

Imagine wanting to send an IP packet from host 'Smith' (with IP address '137.226.4.10'

 

and MAC address '00-10-5A-31-20-DF') to host 'Miller' (with IP address '137.226.4.20'

 

and MAC address '00-10-5A-31-20-EB'). Using the network address and the netmask,

 

host 'Smith' recognizes that host 'Miller' is located in the own network. It therefore has

 

to send the packet through the LAN, directly to host 'Miller'. Unfortunately the LAN

 

interface cannot say: “Send the IP packet to IP address 137.226.4.20”, because the LAN

 

interface only understands MAC addresses.

 

This is why every host has to manage a table that translates IP addresses to MAC

 

addresses. But how do the entries end up in the table? They could be entered manually,

 

but that would not satisfy the objective of making the connection of a new computer to

 

the LAN as easy as possible.

ARP

Therefore the LAN has a special mechanism that automates this process: the Address

 

Resolution Protocol, ARP. The table itself is called the ARP table. Whenever a host does

ELSA MicroLink Cable

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ELSA Cable manual Data transfer within the LAN, Interface only understands MAC addresses, LAN as easy as possible