Need a metaphor for routing

That would be your operator at the switchboard. Take an incomming call and decide:

- if the destination is local, connect it to the extension

- if the destination is remote, patch it through,

- if the destination is unknown, refuse the call

Continuing your telephone analogy, how about a router provides connections to switches in other area codes?

Bert

OK. I buy that then how do I describe a switch?

Hub: Your postal employee puts the same flyer in every house down the street.

Switch: Your postal employee puts the same flyer in every house down the street, one which says, "Please send me a note if you are Agent 86!". So you do, and by looking at the return address on your note, the postal employee knows exactly where to send further messages that s/he receives that are addressed to Agent 86.

Router: Someone wants to send a letter to Agent 86 but doesn't know where you live, so they give the letter to their local postal employee. That employee says, "Ah yes, all Secret Agents live in Toledo" and passes the letter to another employee who is going by the airport. That employee drops the letter at an airline counter, which figures out what the next flight to Toledo is, and waits until the flight is ready and gives the letter to the Captain. The captain flies the letter to Toledo and gives the letter to the airline there. The airline waits for the next time a Toledo postal employee comes by and gives the letter to them. The letter goes to the Toledo central post office, which knows that all of the Secret Agents live in the same neighbourhood, so it dispatches the letter to the postal employee who covers that neighbourhood. The postal employee then sends out the flyer to everyone saying "Let me know if you are Agent 86", and upon receiving the reply is able to deliver the letter to you. Now, each person who passed the letter on to the next person acted as a router -- someone or thing that makes decisions about how to get the letters close enough to the destination that local delivery becomes possible.

[You might want to trim some steps off this last analogy ;-) ]

How about, the switch is like calling an extension and hoping your party is there but a router is like asking the switchboard to find your party?

Actually, IMO that's not quite right.

Whether the IP session is between local or remote hosts, the sender always asks the network to find the destination host. That aspect never changes.

If both ends of the communications link belong to the same IP subnet, ARP (or ND in IPv6) is used to figure out where to send the packet. If the destination IP subnet is different from the sender's IP subnet, then the router becomes involved.

So in fact, the router is the box that ties together different IP subnets. That's all. Very analogous to setting up phone calls between different area codes.

Bert

How about ... a router is a 'cheats' guide to a maze ... at each step the next router knows which path you should take.

BernieM

How is that different from a catenet made of learning bridges (as almost all are these days)?

Bert

of course, telephones themselves are often an example of an "ethernet" "hub". since you can both talk at the same time. analogies aren't necesessary for saying in one port out all ports and rejected by all but the intended receipient. Or in one port out the right port (as expected).

connects telephone switch boards together. like a switch but higher up the tree. e.g. connects the england switchboard to the brazilian switchboard.

routers have an address and it's hierarchical. The brazilian one won't need to know each comp on the england one and vice versa. But they'll know if it's for brazil or for england.

I hope the listener has adequate geography and can put together the concepts of the airline and the postal service. This is another one of those situations where real life is messy not well defined, and in that sense, more complicated than the world of computers.

In computers, analogies are usually a sad excuse

It's not. Your idea is the best.

BernieM

It's difficult to find an exact analogy in the phone system because digital telephone technology merges layers 2 through 4 of the OSI model--there is no distinction between a bridge and a router for that technology. A place where something analogous to a router might be used is in connecting a TV-cable telephone system or a VOIP system to the PSTN.

what about using postal service as analogy ??

A Zip+4 code is awful close to an IP address in its function.

--reed

I don't think it's that difficult, honestly, because after all E.164 numbers can be routed by PNNI, and PNNI is very similar to OSPF.

Routing simply forwards messages from a network with one IP net prefix to a net with a different IP net prefix. Whereas bridging forwards messages within the same IP net prefix. So this is conceptually between area codes vs. within the same area code.

But a significant point is that in BOTH cases, there can be forwarding tables involved. In the old days of broadcast LANs, that was not the case. But nowadays, it is.

For telephones, this has been the case for a very long time. Within one area code, there are many "exchange" numbers, also used in making forwarding decisions. Local calls were not "flooded" to all exchenges within the area code.

Bert

And so you demonstrate how one falls into error by trying to use the telephone system as an analogy. The telephone system has one set of addresses, the phone numbers. LAN technology has two sets, those used by the datalink protocol and those used by the transport protocol. Bridging occurs at the datalink layer and does not _care_ about IP addresses--an Ethernet bridge doesn't care if it's carrying IP traffic, IPX traffic, or random noise, as long as the data is encapsulated in properly formatted Ethernet frames the bridge will transfer it quite happily. The same is true for a Token Ring bridge or an Arcnet bridge.

Again the analogy breaks down--a local exchange is a subnet of a regional subnet that is part of a national subnet. There is only one addressing scheme, not two. A bridge is not working on a subnet, it is working on an independent address space that is mapped to the IP addresses at a level above that at which the bridge operates.

But the forwarding tables for a bridge are using a different addressing scheme from those for a router.

No, only to phones on the same party line.

Now if you really want to confuse the issue then think about IP running on ATM.

Not all that confusing, really, unless you use LANE. If you use RFC

2225, the ATM NSAPA is treated by the IP layer as if it were a MAC address. Just another version of a Link Layer address, as far as IP is concerned. With LANE, you have to use BOTH MAC and NSAP addresses, so that becomes complkicated.

I agree that phone numbers are one numbering scheme vs a separate IP address and MAC address. However, for the purposes of explaining the difference between routing and bridging to neophytes, the important concept is that routing is used when a certain more significant address prefix is changed, and bridging is used when this more significant prefix is not changed. The area code analogy seems obvious. Other analogies I saw seemed quite misleading, on the other hand. But, peace.

Bert

No, the _important_ concept is that bridging occurs at layer 2 and has nothing whatsoever to do with TCP/IP.