>  I've connected the two PCs and they apparently see each other since
> the message is the same on both... "Local area network-limited or no
> connectivity. This problem occurred because the network did not assign
> a network address to the computer".

  No, they're not seeing each other -- they're each saying that they
can't see ANYTHING on the network....

>  In details, the physical addresses are different,

  Each network device has a 48-bit physical address -- usually, this
comes from a ROM on the network adapter -- and within any network
"segment" these MUST be unique.  So having these be different is
correct.

> the subnet mask is the same

  The IP address is always 32 bits, but part of it is the "network
prefix" ("which network/subnetwork is this on?") and the rest identifies
the specific device on that network.  All of the devices on the same
metwork segment SHOULD be in the same subnet, which means they have to
split the IP address at the same point (use the same "subnet mask" --
additionally, they all have to use the same prefix -- the combination
ensures that they all agree about what network they are on.


 > and in the IP addresses, the first two groups of three
> numbers are the same

  IP addresses are 32 bits, but the usual way to write them is to take
each byte (8 bits) and write its value as a number in base 10 (so legal
values are 0 through 255).  As expected (and required if this is going
to work...), your computers are using the same prefix, but distinct host
numbers.

> but on the desktop PC the 3rd and 4th groups have
> three numbers each and the laptop has two each.

  Usually within a small network, the assigned addresses will be fairly
similar -- more similar than this.  Windows has a half-baked system for
computers to assign themselves random host numbers if the network
doesn't, and it sounds like that system is being used at the moment...

>  I found in the internet protocol properties where I can assign Ip
> address, subnet mask and default gateway. Should I fill these in? And
> with what? I tried the "Obtain an IP adress automatically" but saw no
> change. And should I fill in the same numbers on both computers?

  "Obtain addresses automatically" is what you want -- but it's only
half the answer because they need to be obtained from SOMEWHERE.
  The "somewhere" is called a "DHCP server (Dynamic Host Configuration
Protocol), and it's normally a feature of your router.  (The interface
on your router that connects to your ISP probably gets ITS addresses,
etc, from your ISP the same way).

  So you need to configure DHCP on your router.  For a small home
network, with less than 200 computers(!), it's normal to use a subnet
mask of 255.255.255.0, which says "Of 32-bit IP addresses, the first 24
bits are the network prefix, and the last 8 are the host number." 
Normally your prefix needs to be different from any other in the world,
but there are a block of prefixes using this mask that are reserved for
"local" use -- these prefixes can be re-used all over the world.  For
this mask, the IP addresses are 192.168.X.Y, where X is your choice in
the range 0-255 (everything on your network will use this same X), and Y
is another number 0-254 -- you'll pick one value of Y for the LAN
interface on your router, and you'll tell the DHCP setup on the router
the range of values of Y it can hand out to your computers.  (Y can't be
0 or 255, and the router needs one reserved, so DHCP can be told to use
1-100 or 2-10 or whatever.)
  That address assigned to the LAN interface of the router needs to also
be given out as the "gateway" address by DHCP.  With an address and a
mask, each computer can easily tell whether another address is on the
local network segment, or somewhere else.  If it's "somewhere else", the
computer will send its traffic to the gateway on the local network (the
router), and it's up to the gateway to forward it via some other network
(i.e., your ISP)  In this way, a request gets sent from computer to
router, from router to router to router across the Internet, to finally
some server somewhere, and the responses make their way back.

> Also on this screen are options to fill in preferred and alternate
> DNS servers. Do I need this as well?

  DNS, the Domain Name System, translates hostnames like
"www.google.com" into IP addresses.  IF you memorize the addresses of
every computer you want to connect to (and somehow keep them from
changing...), then you don't need this, but generally you do.Typically,
your ISP will have a pair of DNS servers, and often your router will get
their addresses via DHCP.  But you could get away with filling in the
addresses of publicly- accessible DNS servers -- Google's are at 8.8.8.8
and 8.8.4.4.

  If you connect one computer to two networks, it has no way to
automatically know which one it should use to read the Internet.  For
now, just assume that each computer can handle only one network
connection hooked up at any given time.

  And oh yeah -- I mention that that prefix was reserved for "local" use
-- so to reach the Internet, you'll also need to make sure that a roter
feature called NAT (Network Address Translation) is turned on.  Then the
rest of the world won't see your local addresses (which might be the
same as THEY are using), everything will look like the ISP connection of
your router.

  That should be enough to get you well underway.

David Gillett
CISSP CCNP


David Gillett

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