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Technical basics
ELSA MicroLink Cable
61
EB'” in the ARP table and tells its LAN interface: “Send this IP packet to the computer
with the MAC address '00-10-5A-31-20-EB'”.
Data transfer from the LAN onto the Internet
Imagine the second task, sending an IP packet from host 'Smith' to the remote host
'External' with IP address 151.189.12.43. Host 'Smith' compares the IP address with its
network address and realizes that host 'External' is located outside the LAN. So host
'External' can only be reached through the router. Host 'Smith' finds out the MAC
address of the router '00-80-C7-6D-A4-6E' by looking up the router’s IP address in the
ARP table (if necessary another ARP request is made). So host 'Smith' tells its LAN
interface: “Send this IP packet to the computer with the LAN address '00-80-C7-6D-A4-
6E'”. The router extracts the IP packet from the LAN packet and finds out about the IP
address of host 'External'. In the routing table the router then looks for the network
address of this host and thus finds the interface through which to pass on the IP packet.
LAN coupling on MAC basis
You know how LANs simplify the connection of computers to a local network. Nearly all
house networks are thus LAN based. In some cases a LAN is covers such a large area
that the physical characteristics of the cable prohibit the connection of any more
computers. This results in the necessity to couple up several LANs in such a way that
electrically and in terms of the MAC protocol they act as independent LANs, but for the
IP protocol look like one big LAN.
This coupling of LANs is carried out using bridges. A bridge works somewhat like a
router, but uses only MAC addresses for routing, not IP addresses. Since MAC addresses
do not give any information on the structure of the network the way IP addresses do,
every bridge has to know all MAC addresses in the entire LAN.
And so we encounter the same problem that we had with the routers before the
introduction of subnets: As the LAN expands, it will at some point exceed the capacity of
the address tables of the bridges. So one cannot use bridges to connect an infinite
number of LANs. On the other hand, the unstructured MAC addresses allow the bridges
to learn automatically about the location of computers in the network, using the received
packets. This is called an “intelligent bridge”.