It's very simple.  In order to carry out some of my reasearch this summer, I have had to build a "miniature Internet" -- that's what I call this endeavour to people who aren't familiar with the term "Intranet," anyway.  As I anticipate this Intranet of mine growing over time, and as its construction has taught me so much, I decided to document its evolution over the summer.  Again, the main emphasis of my research is supposed to be on protocols, but I can't pretend to NOT be interested in the practicality of learning how to set up a wide area network.
    The page is geared toward the Computer Science type person who knows plenty about networks on a theoretical level, but is interested in, say, the actual command that one enters into a Cisco Router to do all of this stuff.  I know that was my battle cry before this summer!  Now that I possess some of this knowledge, I'd like to "share the wealth."

    When I first walked into the lab, I wanted to assure myself that I could even hook up one computer, much less a whole Intranet.  But plugging into an empty hub as the only solitary connection on an empty Ethernet segment doesn't give you much confidence -- there's no one to ping but yourself!  So, with the help of the guy showing me the lab, I pretended I was a faculty member at UT...

    In the far corner of the room (off in the shadows), there is an Ethernet port which provides access to "UTNet" -- the University's Intranet.  This is the same situation that a new professor faces when he/she enters their new office.  So, what do you need to know to plug your computer into one of these jacks?  Will your computer configure itself "magically?"

    Well, if this connection supported DHCP (Dynamic Host Configuration Protocol), that wouldn't be far from the truth.  It turns out, however, that my lab tour guide had to provide me with the essential info, since there was no DHCP server on that link.  I needed:

• Unique IP address (chosen from a list of "free" addresses that is kept somewhere, I presume)
• Subnet Mask (to know whatt IP addresses will be on this same Ethernet segment and what won't)
• Default Router (to know where to send packets destined for computers NOT on this Ethernet segment)
• DNS Servers (to convert, say, www.utexas.edu, into an IP address)

Figure 1

    All of this plugs right into the TCP/IP setup area in Windows 95, the OS of choice for this first test.  In fact, it worked so well after a quick reboot and plug into the wall, that I decided to tackle my next task.

    For those of you of the Linux frame of mind, I also discovered the essential commands to set up a Linux box on an Ethernet segment.
    Assuming that you have a network card (aka a NIC) that has been recognized by the Linux kernel, you have two main commands you have to use.  First, you use the ifconfig command (as the root user, of course):

% ifconfig eth0 xxx.xxx.xxx.xxx netmask yyy.yyy.yyy.yyy broadcast zzz.zzz.zzz.zzz

where xxx is your IP address, yyy is your subnet mask,  and zzz is your broadcast address (typically the last address in your subnet)

% route add -net xxx.xxx.xxx.xxx netmask yyy.yyy.yyy.yyy

where xxx is your network number -- not the IP address of your machine.  In other words, this is just the network and subnet parts of an IP address that is divided into network, subnet, and host parts. (are you familiar with CIDR?)
where yyy is the subnet mask -- the series of ones followed by all zeros that lets the OS know how many bits are dedicated to network and subnet.

% route add default gw xxx.xxx.xxx.xxx

where xxx is the address of the "router" the OS should send all packets to that are not in your same network and subnet.  If you neglect to enter this command, you will be able to reach all hosts on your local subnet, but will receive a "network unreachable" ICMP error for all other addresses.

    "It's not as hard as it sounds."  Sure, whatever.  Hooking up routers, hubs, and point-to-point links takes some careful thought!

• Green Box - Router
• Brown Box - Hub/Concentrator
• Dark Blue - Workstation
• Light Blue - A Subnet
• Orange Triangle - CSU/DSU to establish WAN links.
• network - The last number in the IPv4 address of this network.  For my network, the first three numbers were 206.77.43.

• Start with a single Ethernet segment with two or more hosts on it.
    Just imagine that your a small, upstart company back before a corporate Internet connection was "required."  If you were moving into a "modern" office building, you'd basically have a bunch of network jacks (which are just scaled-up telephone jacks), with each one having a dedicated wire leading back to a central wiring closet.  Inside this closet, you'd most likely find a large box mounted on the wall with a matrix of numbered network jacks.  Each one of these is a direct connection to a network jack somewhere in the building.
    As network administrator, you would be responsible for acquiring a hub (or concentrator) and a whole bunch of short length of network "patch" cables.  Now, as people move into the office, you get to pretend like your one of those old switchboard operators of the distant past.  When someone moves into an office, you take note of the number of their jack, and you proceed to activate it.  This is accomplished by plugging a patch cable from the jack in the wiring closet into the hub.
    Recall that in 10BaseT Ethernet, everyone shares the same channel for communication.  So, it is completely irrelevant which port on a hub a particular person is plugged into.  The only thing to watch out for might be either the first or last port on the hub; sometimes, the pins on these are different for special reasons.
• Connect the Ethernet to a router.
    Next, imagine your small upstart company wanting to have access to the Internet.  To do this, you will need a router (Cisco is the most popular brand used worldwide).  One of its interfaces will connect to your company's ethernet segment, and another one of its interfaces will connect to the Internet (through a dialup modem account or whatever you'd like to spend your money on).
    Here are the commands you issue to a Cisco router to configure one if its interfaces (let's say ethernet-0)
# configure terminal
(config)# interface e 0
(config-if)# ip address xxx.xxx.xxx.xxx yyy.yyy.yyy.yyy
(config-if)# ^Z
    The xxx.xxx.xxx.xxx is IP address, and yyy.yyy.yyy.yyy is the subnet mask.  Just pretend that this "interface" of the router is a workstation on your ethernet.  You can give it any valid IP address on your network based on your subnet mask.  Often times, the most popular address for a router on an ethernet segment is either the first or last valid address within a subnet.  But it doesn't matter.
    Next, you would configure the interface that connects to your modem or your service provider.  I won't go into that here, but I do want to point out something very important: your numbering scheme HAS to be compatible with the numbering scheme given to you by your service provider!!  What would happen, for example, if your company in Austin picked the same IP address range as a company in China?  Well, as long as you don't hook up to the global Internet, that would be fine.  The problem arises when you get your Internet connection.  I can imagine a case where all of your datagrams get routed over to China, or all of China's get routed to YOU.
    Clearly, if you start out your company not knowing who your service provider (and hence, your network numbers) will be, you will almost suely have to REDO your network numbering plan when you choose a provider.  For what it's worth, that when it's really nice if you are using DHCP or BOOTP to automatically assign configuration parameters to the workstations in your intranet.
• Add more Ethernet segments and routers.
    So let's assume that you haven't pulled all of your hair out yet as a result of hooking your one ethernet up to the Internet.  Now, it's time to hook up more Ethernet segments and connect your private branch offices.  When this day comes for a company, planning is of the UTMOST of importance.  Do you want, say, one central router with point-to-point links going out to all branch offices?  Or do you want several routers distributed around your branch offices with several point-to-point links between each router (the age-old centralized versus distributed debate, basically).  This and other questions will GREATLY impact the type and amount of equipment that you buy.
    The configuration process is conceptually the same as above, though.  Each interface receives a unique IP address on the subnet to which it is plkugged in to.  You have to be carely to not let subnet addresses "overlap" each other (sometimes, routers can catch that kind of error).  A nice diagram outlining your topology is MANDATORY.  In fact, put a nice big, full-wall printout of it in your office.  It doesn't have to be as "fancy" as the ones I have on this page, but it should conceptually contain the same elements.
• Run a routing update protocol.
    Let's say your company has really taken off, and you have, like 5 routers and 20 subnets.  Your routers have to have some way to forward datagram correctly through your network -- the forwarding table.
    It is entirely possible to log in to each of your 5 routers and put in static entries that describe what datagrams should be sent out on what interfaces.  But imagine what a pain that becomes when your topology changes?  Or worse yet, when a link unexpectedly fails and you want to route datagrams through a roundabout, backup link?
    That's where using an automatic routing update protocol comes in handy.  In fact, I can't imagine one single reason why you should NOT use one, unless you just want to make your life difficult.  Whether you use RIP, OSPF, IGRP, EIGRP, or some other fancy one, the results will be the same -- your routers will automatically build their forwarding tables based upon information gathers from other routers in the network.  Amazingly, routing update protocols even handle the dreaded link failure -- some of them can converge to a newly adapted network topology within SECONDS after a failure!

    I wanted to be able to access the public Internet from the lab.  In order to do this, I had to hook one of my router's interfaces up to the central router in the lab.  Then, I'd have to somehow tell this router about my network.

    The gray areas in the picture are the parts I added in.  Also, let's say that I had leased this connection from another company.  I would not be responsible for management of the gray areas -- only the color areas.

    An interesting problem arose at this point.  For my intranet, I was running the RIP routing update protocol, while the network to which the NEL-ATM-ROUTER belonged to was running the IGRP routing protocol.  Would I have to run IGRP to connect to this network?  Theoretically, I knew that I wouldn't have to -- that's the whole idea of "autonomous systems."  The NEL-ATM-ROUTER has no business knowing the internal structure of my intranet -- it should only need to know the address of my "gateway" router.

    Well, after SEVERAL weeks, the solution just "came to me" one day.  This was the line in the NEL-ATM-ROUTER that I needed:

ip address 206.77.43.106 255.255.255.192

    So how did NEl-ATM-ROUTER know whether, say, 206.77.43.89 is a host on the ethernet directly attached to it, or if it needs to forward a datagram for this host my gateway router?  Well, NEL-ATm-ROUTER doesn't care!  Recall, the ARP rpotocol is used to determine the actual data-link layer address of a destination.  When NEL-ATM-ROUTER does an ARP request on this interface, my gateway router will respond with ITS data-link layer address.  As far as NEL-ATM-ROUTER is concerned, it HAS found the destination host.

    Once I realized that all of my crazy, eccentric work would eventually be used by future students, I had to buckle down and make this intranet of mine MAKE SENSE!  So, after a few weeks of happy complacency under the topology described in the previous section, I completely redesigned my intranet (though hints of the old one are still evident).  Here's what I came up with:

    Please don't be intimidated.  This intranet is conceptually the same at everything else I've talked about on this web page.  Each interface of a router is connected to a subnet (once again, 29 bits were used for the network portion, and 3 bits were used for the host portion of the address -- a subnet mask of 255.255.255.248).  Just study this diagram for a few minutes, and it will hopefully be apparent that there's really nothing mystical or uncomprehendable.  This IS an intranet, a wide area network -- the exact same thing that companies worldwide set up for themselves!

    There are a few important things to note here.  First, note that subnetting is being used -- that is, the network segments in this diagram are neither Class A, Class B, nor Class C.  Since this is the case, more or less every address has to have a subnet mask associated with it.  Using the subnet mask, IP can determine if two addresses are on the same subnet or on different networks.  Recall that a subnet is the "finest grained" unit of measurement under IP -- an intranet (or WAN) is simply a bunch of interconnected subnets.

    For simplicity, the same subnet mask is used for all of the subnets in this WAN.  That means that each subnet will have the same number of maximum hosts, which, if you'll notice, is not very many for the mask we are using.  There are only three bits for the "host" portion!  But that will work fine for this setup -- it would be pretty useless if we were really interconnecting real offices between cities (you couldn't have more than a handful of workstations in each office!)

    Also, notice that all of the small subnets (such as .80, .88, .96, etc.) can be grouped into a supernet of 206.77.43.64.  This supernet has a subnet mask that allocates 26 bits and 6 bits to the network and host portion, respectively.  This is the beauty of hierarchial addressing -- my entire intranet, with its 7 subnets and four routers, can be completely represented to the outside world as this one network address -- the rest of the world doesn't need to know how I chose to subnet my intranet!
 
    So, that's it!  My intranet, over the entire summer, slowly became the sophistocated, fancy picture that you see above.  If you're interested in what this final product will be used for, check this out.