[Sussex] Re: Sourcing list of hardware

[Steven Dobson]

Fay

On Wed, 2006-10-25 at 00:24 +0100, Fay Zee wrote:
> For example, at home, I use a network switch upstairs, attached to my
> firewall machine, then when anyone comes I throw a cable down the
> stairs and hang another network switch off it. Could I simply do that
> to start with?

Yes.

>  Have, say, one 4-port switch, then four more 4-port
> switches hanging off it? That would give 16 PCs access to the
> Internet.

No.  If the switches only have four ports each then one of the ports on
each of the "four more 4-port" switches would need to be connected back
to the "master"
switch.  So you would only have 12 ports free for computers.

If the 4-port switches had a fifth "up-link" port then yes you would
have capacity for 16 computers.

> There are also 12 hotdesks, so to accommodate these extra
> connections, how about yet more switches...? Like I saw at a HantSLUG
> meeting? Or is it truly best to get a rack-mountable at the outset?

Yes.  For small networks this is not a problem.  However, on a LAN
computers send data not between IP addresses but between MAC addresses.
The MAC address is the hardware address of network card in your
computer.  IP addresses are an abstraction built on top of MAC
addresses.

Computers find out the mapping from IP to MAC address by sending out APR
request which are broadcast to every computer on the LAN.  The computer
with the IP address then sends back a responce and the computer builds
up a table of mapping.
 
If you run the tcpdump command as root you can see these ARP messages:  
   # tcpdump -i eth0 -n -q -l arp
   01:43:16.705903 arp who-has 10.100.1.5 tell 10.100.1.8
   01:43:16.706133 arp reply 10.100.1.5 is-at 02:60:8c:9f:95:c6
   01:43:21.708194 arp who-has 10.100.1.8 tell 10.100.1.5
   01:43:21.708210 arp reply 10.100.1.8 is-at 00:15:f2:18:f1:86

Switches don't have MAC addresses only computers do (well it is the
Network Interface Card [NIC] that does - so a computer with more than
one NIC will have multiple MAC addresses - your firewall has two).
Switches listen for these ARP messages and then assocated the ports with
the MAC address.  For the ports to which a computer is connected this is
a simple one to one mapping.  But for those ports that are connect to
other switches then a number of MAC addresses have to be assocated with
that port.  This takes memory and switches have a limited amount of
memory.  There is a limit - but were talking about hundreds of computers
here.

> I'm glad John mentioned that VoIP use would make a difference, as at
> some point in the future I intend to get Asterisk up and working for
> all the workstations, which of course will all run Linux. I had never
> heard of SDSL but have now read up on it. I am with Zen Internet,
> which has proved incredibly reliable. I'd like to remain with them,
> budget permitting, but have checked their web site and found it's
> quite expensive: http://www.zenbroadband.com/ML_Business.aspx.

I wouldn't worry about SDSL for VoIP support.  256Kb upload speed of an
ADSL connection will support (I believe) 3 or more concurrent
conversations.  If you only want VoIP to talk to one or two friends at a
time then you should be okay with what you've got.

Steve