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Routing and Addressing
Routers conspire to deliver packets in the Internet.
Everything in any part of the Internet that wants to be reached must
have an address. The address tells the computers in the Internet
(hosts and routers) where something is topologically. Thus the address
is also hierarchical. My computer's address is
128.16.8.88. We asked our Internet provider
for a network number.
We were given the number 128.16.x.y.
We could fill in the x and y how we liked, to number the computers on
our network. We divided our computers into groups on different LAN
segments, and numbered the segments 1-256 (x), and then the hosts
1-256 (y) on each segment. When your organisation asks for a number
for its net, it will be asked how many computers it has, and assigned
a network number big enough to accommodate that number of computers.
Nowadays, if you have a large network, you will be given a number of
numbers!
Everything in the Internet must be reachable. The route to a host will
traverse one or more networks. The easiest way to picture a route is
by thinking of how a letter to a friend in a foreign country gets there.
You post the letter in a postbox. It is picked up by a postman (LAN),
and taken to a sorting office (router). There, the sorter looks at the
address, and sees that the letter is for another country, and sends it
to the sorting office for international mail. This then carries out a
similar procedure. And so on, until the letter gets to its
destination. If the letter was for the same 'network' then it would
get immediately locally delivered. Notice the fact that all the
routers (sorting offices) don't have to know all the details about
everywhere, just about the next hop to go to. Notice the fact that the
routers (sorting offices) have to consult tables of where to go next
(e.g. international sorting office). Routers chatter to each other all
the time figuring out the best (or even just usable) routes to places.
Note sure this is the right place for this - mjh
The way to picture this is to imagine a road system with a person
standing at every intersection who is working for the Road Observance
Brigade. This person (Rob) reads the road names of the roads meeting
at the intersection, and writes them down on a card, with the number 0
after each name. Every few minutes, Rob
holds up the card to any neighbour standing down the road at the next
intersection. If they are doing the same, Rob writes down their list
of names, but adds 1 to the numbers read off the other card. After a
while, Rob is now telling people about the neighbours roads several
roads away! Of course, Rob might get two ways to get somewhere! Then,
he crosses out the one with the larger number.
To support multicast, routers need to know where the recipients are
(or possibly, where they are not!). The first step in this is for
routers to take part in the IGMP.
Next: Multicast Routing
Up: Multicast
Previous: Host Functions
Jon CROWCROFT
1998-12-03