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Technical basics
ELSA MicroLink Cable
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i.e. all it has to remember is the network address and the netmask of the subnet in 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.
A 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.
The 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
IP host
in the LAN
LAN with router function
IP host
in the LAN
The router in the
LAN:
knows only the
address of the LAN