[AccessD] OT: Hubs attached to Routers

Drew Wutka DWUTKA at marlow.com
Wed Feb 12 00:39:00 CST 2003


Ooops, sorry for repeating then....I'm stuck in the mire of posts from the
last few days....I just decided to start at the top now to see what's posted
to my posts....

Drew

-----Original Message-----
From: John W. Colby [mailto:jcolby at colbyconsulting.com]
Sent: Tuesday, February 11, 2003 10:55 PM
To: accessd at databaseadvisors.com
Subject: RE: [AccessD] OT: Hubs attached to Routers


Drew,

Much appreciated.  It all pretty much matches what I sorta knew.

Thanks.

John W. Colby
Colby Consulting
www.ColbyConsulting.com

-----Original Message-----
From: accessd-admin at databaseadvisors.com
[mailto:accessd-admin at databaseadvisors.com]On Behalf Of Drew Wutka
Sent: Tuesday, February 11, 2003 11:42 PM
To: 'accessd at databaseadvisors.com'
Subject: RE: [AccessD] OT: Hubs attached to Routers


John, Bob already replied to this, but I'd like to clarify a few things.
This is not entirely OT though, because the biggest, baddest wolf, when it
comes to Access db corruption is bad networking.

A while back, I posted a few emails about protocols.  I know I was harped on
for those posts, but they explained some of this.  Every network card has a
MAC address, which is supposed to be a completely unique 'key'.  (They are
nowadays, but a while back there were some goofs.).  However, MAC addresses
are pretty big, and have no rhyme or reason to them (unless you got a
particular batch that have their MAC addresses sequential).  Therefore, IP
Addresses are used to present a more orderly system.  When you plug a NIC
into a switch or hub, your NIC communicates at a very low level, this is the
'Ethernet' protocol.  This protocol let's the switch/hub how to communicate
back and forth to your NIC.  The IP address is kind of like your Zip code.
It just provides an easier method of finding your NIC.  There are older
'card level' protocols, like Token Ring, etc.

Now, get the term Router out of your vocabulary, when talking about LAN's.
A Router is a highly misused term, essentially because every router I have
ever seen is also a switch, (except for a Windows 2000 Server 'software
based' router).  A router does not allow communication between computers on
a LAN.  (Sort of).  A Router allows communication between subnets.  (You can
have multiple subnets on a LAN, so it 'can' allow communication within a
LAN, but it's a matter of scaling).

Hubs and switches are the 'controllers' within a LAN.  You are right about
Hubs, they are essentially one big cross connect, like a room full of
people.  Everyone can talk at the same time, but communications will get
garbled then.  A Switch actually  remembers who is where, so it can direct
communications, so it is more like an office phone system.  Everyone can
talk at the same time, and the communication is 'directed' to and from the
'talkers', instead of clashing with everyone else on the system.  To do
this, switches build what is called a NAT table.  NAT is 'Network Address
Translations'.  Last night was a prime example to explain how a NAT table
works.  I turned off all of my computers Friday night (because I was moving
this weekend).  Last night (Monday), I finally set my computers back up, in
my new place.  In process, I reconnected everything to my 3Com switch.  I
was able to communicate between my various machines (Server, Print Server,
Laptop, and Desktop), however, communications were a bit sluggish at first,
but within 30 to 60 minutes, everything was running smoothly.  This is
because my switch had to rebuild it's NAT tables.  It was remembering where
I had everything plugged in before, so when I tried reaching a machine after
the startup, it tried sending the packets to where it 'thought' the
recipient was.  However, since I know I didn't plug everything in the same
way, it was having problems doing this.  It can't just immediately rebuild
it's NAT tables, because doing so would create a lot of overhead if a
computer was simply off, or there was a bad physical connection.  So
essentially, the switch 'relearned' the network, and everything started
rolling smoothly again.

With a Hub, the performance would have been the same no matter where I
plugged things in, because there are no NAT tables to help the packets find
there way.

Just an FYI, TCP/IP is a packeted protocol, which means that it sends it
data in tiny chunks.  Those chunks are complete independant of their
purpose, in other words, if you are sending a Get or Put statement, to a
network file share, you may have the command split up between several
packets.  TCP/IP splits the data/commands up before it sends it out, and
puts them back together before it processes a command.  Thus, the packets
have 'headers' that have both sender and receiver addresses, along with the
order in which they need to be processed (also parity information, so they
can be 'validated'.).  This is important to understand, because with a
switch, when your NAT tables are built, the chances of a packet stream being
interupted is pretty slim from a collision point of view, where as with a
HUB, if you are trying to do something that is constantly interupted by
another communication, you will get varying delays in communication based
upon when the packets are interrupted.

Okay, sorry for the lengthy email, but one last thing.  I said to forget
routers when dealing with a LAN.  You mentioned 192.168.xxx.xxx.  192.168
and 10.10 are 'reserved' LAN IP Addresses.  Every IP address must be unique
within their 'subnet' or network.  (MAC addresses must be entirely unique,
since there is no way of knowing whether or not two NICs are going to be put
in communication with each other).  There are other 'reserved' IP Addresses,
however, on the 'big' LAN, every IP Address is unique.  So how can you have
2 or more LAN's, both with 192.168.0.2 IP Addresses in them, communicating
to the internet, or even with each other?  A Router!  A Router learns both
of it's 'sides'.  Your Gateway setting, in TCP/IP properties, is the address
to the router on your LAN.  Thus, when a computer tries to 'contact' a
machine that is not within it's subnet (determined by it's subnet mask...),
it sends the communication to the router, which looks on it's other side.

I hope this makes sense...I apologize for the length, but this is hard to
explain in a few sentences.

Drew

-----Original Message-----
From: John W. Colby [mailto:jcolby at colbyconsulting.com]
Sent: Friday, February 07, 2003 4:54 PM
To: AccessD
Subject: [AccessD] OT: Hubs attached to Routers


My understanding of things network is minimal although I do understand
Electronics.  However...

My understanding of a Router is that it is a cross point switch matrix.  It
literally connects one input to one output.  Thus traffic between two jacks
is not imposing on any other jacks (physical connectors), i.e. port 1 can
talk with port 2 (or any other port) at 100 mbps, at the same instant in
time that port 3 is talking to port 4 at 100 mbps.  There is no traffic
collisions since there is no connection between the circuits supporting the
two conversations.  IOW any port can hold a private conversation with any
other port.  Since this is the case, any remaining port can hold another
separate conversation with any remaining port.  Etc. Etc. until all ports
are busy.  It appears then that a 4 port router can create two separate
private conversations, an 8 port router can create 4 separate circuits etc.

How it does this I never understood, but that was the way it was explained
to me.

A Hub on the other hand simply connects all of the ports together all of the
time.  Therefore while Port 1 is talking to Port 2, no other ports can talk
without traffic collisions.

Before I go on, I need to ask if this is truly the case?

I would also appreciate a simple explanation of how the router knows that
data coming in on port 1 is "going to" the machine on port X.  I have to
assume that (using TCP/IP) each machine has a 192.168.x.x address (or
something similar) and that address is part of the packet.  The router knows
which machine is associated with each of those addresses, and therefore
simply closes a switch to "route" the packet to the right place?

Next, what happens when I connect a hub to port 1 (for example)?

My understanding is that all machines on the hub can only talk to one
machine at a time, but that if you take that into account, any machine on
the hub could still talk to port 2, while port 3 was talking to port 4.

My question really is, am I slowing down the network for ALL other devices
on the router?  My understanding is no.  However all machines on the HUB
share a connection to the router.  Kind of like a party line (in telephone
terminology).  The hub is a party line, the router is a private line?



John W. Colby
Colby Consulting
www.ColbyConsulting.com


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