On 11 Aug 2001, at 10:41, Harsha Godavari wrote:

> .... I need to know the software side of it, you know all those
> protocols and binding things  and how do they get all those
> aaaa.bbbb.cccc.dddd's? How does one set upt a "subnet mask".

  IP addresses are a lot like phone numbers.

  IP addresses and subnet masks are 32-bit numbers.  They're normally
written by splitting them into groups of 8 bits ("octets"), and
interpreting each octet as a number between 0 and 255.

  In general, a (routable, layer 3) network address needs to consist
of two parts:  a network address (area code) and a host address
(local number).  What's different about IP (TCP/IP) is that instead
of two separate fields (like IPX, AppleTalk, and ordinary phone
numbers), these two parts are smooshed together into a single number.
  If you look at a (valid) subnet mask in binary, you'll find that it
consists of a bunch of '1' bits *in a row* and then the remainder are
all '0'.  This tells the software which part of the IP address is the
network part (the '1's) and which part is the host part (the '0's).
A short subnet mask allows for only a few networks, containing lots
of hosts; a longer mask allows for lots of different network
addresses, but fewer hosts on each network.

  Just as the phone company will assign you a phone number, your ISP
will assign you an address, or a group of addresses.
  What if you have more machines than your ISP will give you
addresses for? Just as some office phone systems provide for
extension numbers, some specific IP address ranges are reserved for
local/private use.  A gateway performing NAT or PAT address
translation allows machines on these addresses to use the gateway's
public address for "outgoing calls".
  For a small home network, the preferred private range is
192.168.x.y, with a subnet mask of 255.255.255.0.  x can be anything
from 0 to 255; experience suggests that about 70% of people pick 1
and about 29% pick 0, and almost nobody picks anything else.
Whatever you pick, it will be the same for all of your internal
machines.  (Unless your internal network is split into subnets, with
one or more routers.  Mine is, but that's probably unusual.)
  y is the host address, and will need to be different on each
machine.  And there's a special rule which says you can't assign a
host address that is all '0's (0, in this case) or all '1's (255).
It's pretty common to make the gateway's address be .1 (so it's easy
to remember), and number the other machines up from there.
  [The gateway is going to have two addresses, one assigned by the
ISP, and one assigned by you.  These will normally be on two
different interfaces -- the private one on the NIC that connects to
your hub/switch/other internal machines, and the public one on the
NIC that connects to your DSL router, cable modem, or whatever.  ICS,
WinGate, or whatever you're running on that machine to provide NAT
will pass traffic between the two networks.]

  So:  What you will need to configure on each machine is (1) it's
full IP address, (2) the subnet mask you're using, so it knows how
much of the address is network and how much is host, and (3) the
private address of the gateway.
  When a host wants to contact some other machine, it will use the
subnet mask to determine if they have the same network address or
not.  If they're the same, it will connect to that machine on the
local net (a "local call").  If they're different, it will send its
traffic to the gateway, who will pass it on to the ISP and usuaully,
eventually, to somebody (a "router") who knows where the destination
network is (a "long distance" call).

  Unless a host is acting as a gateway/router, those three parameters
are all the addressing it needs to be able to "dial by number".  The
other things you might need to set are addresses of DNS servers
(again, usually provided by your ISP); think of these as "directory
assistance" which allow your machines to contact others by *name*
rather than number.  (WINS provides a similar function for multi-
subnet Windows networks -- home users don't need it.)

David Gillett

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