Understanding TCP/IP
server. For systems which require other protocols, such as older versions of Novell
NetWare, it is possible to tunnel IP over other protocols for example, by
encapsulating IP packets inside IPX packets. If the client machine's network stack
hides this behind a standard API, such as Windows' WINSOCK, IP based appli
cations can run unmodified. As all major client and server OSes today support IP
natively, even alongside other protocols, there's little reason to do this, although
it may be used for making secure, encrypted connections over public networks.
How It Works
The snag is that building an IP network requires significantly more planning than
when using most other protocols. IP was developed in the 1960s for linking
disparate networks separate in both a geographical sense and in the sense of
running different, incompatible systems. Protocols such as IPX and AppleTalk,
intended for small LANs, are inherently simpler.
Addresses
The first issue is IP addressing. Each device on an IP network requires a unique
address. Unlike in other protocols, this is not automatically generated from the
hardware (MAC) address; it must be manually assigned. The word device here
is important. It does not mean each computer; IP addresses go by network port.
For example, a server with two Ethernet cards (such as a firewall) would need
two addresses, one per interface. Similarly, a machine with both a network card
and a modem (or terminal adapter) requires addresses for both. To make matters
even worse, it's possible to give one port multiple addresses, a technique called
multihoming . For instance, this allows a single machine to host several separate
Web sites; each hostname points to a different address, but all refer to the same
machine.
The address is divided into two parts: the network number and the host (or
machine) number. All hosts on the same IP network must share the same network
number, and no two hosts may share the same host number.
Subnet Masks
Alongside the address, each port requires a subnet mask. This value is used to
split the complete address into network and host parts; in other words, to
determine whether other IP addresses are on the local network or a remote one.
These two values are the absolute minimum. Using these, a machine will be able
to communicate with others on the local network if the other machine's IP address
is known. Additional information is usually required, though, to be able to access
nodes on other networks, to access machines by name rather than number, and
The standardisation
so on.
on Internet
Gateways
communications
For direct access to networks beyond the current one (which isn't always re
quired), each machine must be told the IP address of the router (or gateway) that
protocols and the fact
connects the local network with the wider world.
that much Internet
Name Servers
For a small, server based network with only one or two servers, access to them
software is free
by their numeric IP address may be sufficient, but usually it's desirable to use
names instead. The most basic way of doing this is via a local configuration file
means that, even for
called hosts. As a minimum, this contains a pair of entries per line, separated by
spaces; first the address, then the corresponding name. However, for all but the
purely internal
systems, businesses
8
16
24
32
Class A
Network Number
Host number
Host number
Host number
can reap significant
1. 127.
0. 255.
0. 255.
0. 255
Class B
Network Number
Network Number
Host number
Host number
cost savings by using
128. 191.
0. 255.
0. 255.
0. 255
Internet
Class C
Network Number
Network Number
Network Number
Host number
192. 223.
0. 255.
0. 255.
0. 255
technologies.
Figure 2 Network and host numbers by class.
Issue 126:January 2001
PC Network Advisor
File: T1838.2
page 10
Tutorial:Internet
www.pcnetworkadvisor.com
< Next page >
New! The best sites for quality inkjet printer cartridges and the best sites for cheap inkjet cartridges