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

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

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