TCP/IP basics

Let me try to give you a short (it will not be very short) and simple introductionto
TCP/IP. I will try to keep it as simple as possible
(it may to be too simple forsome of you, but if you need all the
TCP/IP details, you need to go to your local bookstore and buy
on of these books with 1000+ pages on TCP/IP). I will also
not bother you with the history of TCP/IP and the Internet,
unless required for the understanding of the setup.

IP-address:
Each Ethernet board worldwide has a unique Ethernet-address,
it is a 48 bit number(the first 24 bits indicate the manufacturer,
the last 24 bits are a unique number for each Ethernet board/
controller-chip assigned by the manufacturer).This is also called the MAC-address.

When systems on a local area network ("LAN") are configured with NetBEUI or
IPX/SPX protocol, they use these hardware-addresses to identify
each other, so there is no need to define manually a network address.
But TCP/IP was designed as a Wide-area-network ("WAN"), able to continue
to function, even if part of the network was not operating ( damaged or destroyed).

TCP/IP uses IP-addresses, which are 32-bit numbers. To make it easier to memorize
such IP-addresses, they are usually expressed as 4 8-bit numbers
(example: 192.168.10.1), where each of the 4 numbers is within the range of '0' to '255'
(there are restriction on using '0' and '255', avoid using them.).
When setting up a small private network, you are free to use ANY IP-address, however,
when you are connected to a company network, you need to ask the Network-administrator
to assign you an IP-address. And if you are connected to the Internet, your ISP
(Internet Service Provider) will assign an IP-address to you.

Even if a network is NOT connected to the Internet, it has become custom to use on
private networks a range of IP-addresses, which are reserved for private networks
(that makes it later possible to connect your private network to the Internet without
having to re-configure everything). The reserved IP-address is: 192.168.x.y, where
x=same number on all systems and y=different/unique number on all systems.
A small network of 3 systems would use:

You configure this IP-address in the properties of the TCP/IP-protocol:

(For now, simply enter as 'Subnet Mask" 255.255.255.0, it will be explained later
in this document)
That's it, if you just like to connect systems on a small network, the network should
work and you can test the Connection using the TCP/IP ping.

On a small network, you can still memorize the IP-addresses used, but if your network
grows to 50+ systems, it becomes a serious management job. But TCP/IP offers some
help by allowing to configure it to:
"obtain an IP address automatically":

To be able to make this automatic assignment, there needs to be now on the network
a database, keeping track of possible IP-addresses and to whom these addresses have
been assigned:

DHCP (Dynamic Host Configuration Protocol)
On bootup, the system sends out a call on the network to find a DHCP-server,
which assigns an IP-address to such a system. The IP-addresses are usually
assigned NOT permanently, but for a specific time (could be days, weeks, months
or on Internet-connections just for the ONE connection). If the system contacts the
DHCP-server again during this time, the 'lease' on the IP-address is extended. But if you
come back from a long vacation, your 'lease' of the IP-address may have expired,
that IP-address may have been assigned now to somebody else, and you/your compute
r get now assigned a new IP-address.
Windows95 itself does NOT include any DHCP-server, you need to connect to a
Windows NT (or similar class) server , which is configured as DHCP-server.

Microsoft supplies now with Windows98/ME and with Windows 2000/XP a feature for IP-Auto-Configuration without a DHCP-server on the network.

DHCP-server may also be buildin to some other products ( example : s
oftware Router for
Internet Connection Sharing ) .

If you are using/intend to use "obtain an IP address automatically", please do NOT reply
on it without verifying, that you did get an IP-address assigned.

Looks simple until now ? Actually there is already a lot more 'hidden' actions:
The systems have IP-addresses, but Ethernet-boards ONLY know their Ethernet-
address,so as soon as a TCP/IP configured system is switched on, it is advertising its presence onto the network:" Hey, I am alive, my Ethernet address is '08000b 0a0238' and myIP-address is '192.168.10.2' ". , and each TCP/IP system on the network
builds up a table with all this information, which is usually checked/verified in time-
intervals of 15 min.

If your system needs now to communicate with a station, for which it does NOT have
an
entry in its table of IP/Ethernet-Addresses, it sends out a search-message to
everybody ("Broadcast-Message") like: " Hey, I like to communicate with the IP-address '192.168.10.4', but I do NOT know your Ethernet-Address. Please, identify yourself". This causes the system
with the requested IP-address to send out its advertising again.

These processes are called ARP (Address Resolution Protocol) and RARP
(Reversed Address Resolution Protocol).

This ARP/RARP works fine on a local-area-network (on an Ethernet network), but
will NOT work for Internet communications, because:
- the Database of Ethernet-to-IP-address would need to have 10+ Million entries
- the Internet would only be busy with ARP/RARP.

Gateway/Router:
To connect a TCP/IP local-area-network to another TCP/IP LAN (which could be
the complete Internet) or via a Wide-Area-Network (WAN), you need now a device
called : Gateway or Router


You need to tell Windows95 about the Gateway in the TCP/IP-properties:

Now, also the 'Subnet-Mask', which is usually '255.255.255.0', becomes important:
ifyou now like to connect to 207.68.137.53 (which is the Website of Microsoft), TCP/IP
checks your own IP-address and the IP-address of the destination against the Subnet-mask
Lets do that comparison on a binary level:

TCP/IP compare now the part of the addresses, defined by the '1's in the subnet-mask
( or simple: the part of the IP-addresses, where the subnet-mask is 255 ):
xxx indicates, that this part of an IP-address does not matter anymore :

if your system connects to another system on the same network (like a local server,
inthis example at 192.168.10.10) , that part of the address (the first 24-bits in this example)are the same, ( 192.168.10.xxx ) so TCP/IP looks up the Ethernet address
in its ARP table and connects directly to that system.

But if there is a difference in these 24-bits, then TCP/IP connects to the Gateway
(in this example: 192.168.10.20), and it is now the job of the Gateway to establish
somehow the connection to the destination system (somewhere inside that Internet 'cloud').
The Gateway/Router keeps for that purpose special tables and passed on the
request to the next router, which itself goes to the next, which itself goes to the next,....., until you reach the destination.

Each Router/gateway on the network (which could be the Internet or a
Wide-Area-Network WAN)
passes on the message, until it reaches its destination, and the reply comes back the same way

all lines starting with an '#' are comment lines.
The formatting of 'hosts' and 'lmhosts' is the same:
IP-address, some spaces, computername
example:

When to use HOSTS and when to use LMHOSTS ?
That is a confusing subject: having 2 files with a very similar job.
HOSTS is read by basic TCP/IP software (ping, ftp, ......)
LMHOSTS is used by the Microsoft Networking/Client/Workgroup management.
If systems are on the same cable segment, the system broadcast their presence
and find each other automatically, no need to enter anything in LMHOSTS.
However, such broadcast-packets to NOT get routed. Adding then the IP-address
manually in LMHOSTS makes the system aware about a system on different segments.
NOTE: LMHOSTS originates from "Lan Manager HOSTS", a name from the history
of Microsoft networks.
An example for using LMHOSTS : Connection via a Router to a NT Domain Server

DNS:
Too much work to typ these IP-addresses ?
Looks like another item for automation, and exactly that is DNS : Domain Name
Service:it allows to use names instead of IP-addresses, but you need to configure it as part of the TCP/IP-properties:

If you now define an address ( like: www.microsoft.com or someserver.com in the
picture below)
TCP/IP will make:
1: a call out to a DNS-server, asking for the IP of someserver.com
2: the DNS server will reply with the IP-address (in this example 192.5.6.111)
3: TCP/IP makes now the connection to the requested server someserver.com,
using the IP-address 192.5.6.111 :
Port:
TCP/IP is a complex protocol, offering multiple services (especially on the Internet),
like:
- HTTP (=Web-Browsing)
- FTP
- e-mail
- file sharing
For each of these services, a port is used for the specific type of communication
(advanced TCP/IP)

It is possible to have Multiple IP-addresses on a Network Card.

That's it for a basic course on TCP/IP. .