...and how does each computer end up talking to the world?
Well, the computer has as ethernet port on it, into which you plug a patch cord, which plugs into the ethernet jack on the wall. There's a wire behind that thing that runs back to the wiring closet where it is attached to the patch panel. Another patch cord goes from there to a hub or switch, which connects to a bunch of other hubs and switches. Eventually, it connects to a router, which is connected to a bunch of other routers. Finally, somewhere out there, a router is hooked to a switch that's hooked to the server you're talking to.
Anything that is to talk to a wired network needs a means of connecting to it: that's your network connector. In the ethernet world (that's the type of networking that just about everybody uses these days), the connector on your computer is called your "ethernet port", while, for reasons lost to time, the connector on the wall is called the "ethernet jack".
Here's a computer's ethernet port (left/top) and an ethernet jack from a wall:
Behind the faceplate of the jack on the wall is a wire that runs back to a central area in each building or on each floor called a network closet. The face plate, jack, and wire collectively are known as an "ethernet drop" or a "network drop".
We have something like 5000 ethernet drops at Morehouse.
The wire leading from your computer to the wall is called a "patch cable", an "ethernet cord", or some variation on those things. Those come in lengths from 3 feet to about 100 feet, have 8 wires in them, and look a bit like oversized phone cords. They come in a variety of colors, too; there's no set meaning for the colors (different colors of cable are the same except for their hue).
Here's a coiled-up cable that's probably 7 feet long and a close-up of
the connector on the end of such a cable.
In each building or on each floor is a gathering place for wiring known as the "network closet", "wiring closet", or some such, even though it's not always an actual closet. In there you'll find a 19" data rack, which is just a piece of metal with holes drilled in it that you can use to secure other devices (like hubs) to the wall or to each other; it's really just a fancy shelf. Attached to that are some patch panels, which are where you'll find the other end of the wire from the back of your ethernet jack. It's plugged into the back of the patch panel, which provides one ethernet jack per drop on its front.
Here (left) is the back of a patch panel and (right) the corresponding
front. Each of the blue cables at the left goes to one jack on the
wall somewhere. Each cable also contains 8 wires, just like a patch cord.
These days, the terms "hub" and "switch" are used to refer to the same items: electronic devices that provide ethernet signals. These provide the most basic level of connectivity, namely electricity. Technically, switches are smarter than hubs, but that distinction is best left to the nerds.
On hubs and switches you will find a number of ports, each of which can talk to one computer by way of a patch cord. If you have more computers than you have ports, you need a switch with more ports, or maybe more switches. Some are the size of a pack of playing cards and have 4 ports on them; others have 24 ports or more and are meant to be grouped together.
The basic job of a hub or switch is to let multiple computers speak directly to each other. Two computers hooked to the same switch typically can talk to each other without interference. That's good if it's you're goal; it's bad if you want some security in the mix.
In the pictures below, the itsy bitsy hub is the size of a pack of cards,
while each of the big hubs in the stack is about 2 inches high, 16 inches
wide, and 12 inches deep. That big hub stack can handle about 180 users
(some ports are lost to interconnections); many of our dorms have piles
A wireless access point is basically a hub with no wires: it uses radio signals to do its talking. Each one can handle some number of clients, often about 30, much like a hub. They come in lots of sizes and shapes and have many different feature sets: those you'd use at home are completely unsuited for use in a campus environment (and vice versa).
A home-style Linksys unit is on the left; an enterprise-type Cisco is
on the right.
The term "router" refers to an electronic device that knows how to take network traffic from one network and stick it on another one. In these terms, a network is a collection of hubs and switches all plugged into each other. Networks like our have very few routers, often numbering in the single digits; we have 2.
For instance, at Morehouse, we have one group of hubs and switches (and no routers) that constitutes the Faculty and Staff network, and a separate collection of hubs and switches that constitutes the student Residential network. They're as separate as Atlanta and Savannah... We use a router to connect the two together.
Routers are sort of like the multi-operator guided tours one often finds in foreign countries or domestic tourism outfits: you contact one, tell it where you want to go, and get handed off from taxi to train to taxi for a while until you get there. Of course, in networking terms, your computer knows how to start that process.
Here's someone's glamour shot of a medium-big Cisco router (19" wide,
8" tall, about 24" deep). Notice that it has several cards in it that
have different connectors on them: one thing routers can do is talk
to diverse networks, so this one might speak ethernet, Token Ring, ATM,
and who knows what else.
Servers are nothing more than computers, sometimes with overbuilt parts in them, that have been told to provide some kind of service. They might serve files, accept print jobs, push out web pages, scan email, handle database transactions, or whatever, but at their hearts, they're computers. If we put the right software on it, the computer on your desk would be a server. As with routers, there tend to be relatively few of these in an organization. Morehouse has 30 or 40.
In the bad old days, servers really were plain old computers. Here's a
server room from about 1990 (by the look of it):
Nothing in the data network is less well-understood than the firewall. It's little more than a router that sometimes says "no".
Normally, when you try to make a network connection to something (Yahoo!'s main web page, for instance), the routers along the way try to help you out with that by passing your traffic to its destination and back. If one of those routers is a firewall, it'll pass your traffic just like a normal router, unless it has been configured not to.
Some firewalls disallow traffic based on static rules like "never let anybody talk to Google"; others try to analyze the traffic as it goes by so they can have rules like "slow down anybody who's running Kazaa". This is hideously complicated to implement, but the basic idea remains the same: act on requests to pass traffic by either doing it, not doing it, or doing it a special way.
Firewalls can look just like routers. They can be computers, built on normal hardware or on server hardware. At Morehouse, we use plain old desktop computers running highly customized software for our firewalls and are moving to slimmer, rack-mounted computers like those seen above.
Hope that helps!