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Before diving into the delight of sub-networking, we need to establish
some IP number basics.
First of all, let's clear up a basic cause of misunderstanding - IP
numbers are not assigned to hosts. IP numbers are assigned to
network interfaces on hosts.
Eh - what's that?
Whilst many (if not most) computers on an IP network will possess a
single network interface (and have a single IP number as a consequence),
this is not the only way things happen. Computers and other devices can
have several (if not many) network interfaces - and each interface has
its own IP number.
So a device with 6 active interfaces (such as a router) will have 6 IP
numbers - one for each interface to each network to which it is connected. The
reason for this becomes clear when we look at an IP network!
Despite this, most people refer to host addresses when referring to an
IP number. Just remember, this is simply shorthand for the IP number
of this particular interface on this host. Many (if not the majority)
of devices on the Internet have only a single interface and thus a
single IP number.
In the current (IPv4) implementation of IP numbers, IP numbers consist
of 4 (8 bit) bytes - giving a total of 32 bits of available information.
This results in numbers that are rather large (even when written in
decimal notation). So for readability (and organisational reasons) IP
numbers are usually written in the 'dotted quad' format. The IP number
192.168.1.24
is an example of this - 4 (decimal) numbers separated by (.) dots.
As each one of the four numbers is the decimal representation of an 8
bit byte, each of the 4 numbers can range from 0 to 255 (that is take on
256 unique values - remember, zero is a value too).
In addition, part of the IP number of a host identifies the network on
which the host resides, the remaining 'bits' of the IP number identify
the host (oops - network interface) itself. Exactly how many bits are
used by the network ID and how many are available to identify hosts
(interfaces) on that network is determined by the network 'class'.
There are three classes of IP numbers
- Class A IP network numbers use the leftmost 8 bits (the leftmost
of the dotted quads) to identify the network, leaving 24 bits (the
remaining three dotted quads) to identify host interfaces on that
network.
Class A addresses always have the leftmost bit of the leftmost
byte a zero - that is a decimal value of 0 to 127 for the first dotted
quad. So there are a maximum of 128 class A network numbers
available, with each one containing up to 33,554,430 possible
interfaces.
However, the networks 0.0.0.0 (known as the default route) and 127.0.0.0
(the loop back network) have special meanings and are not available for
use to identify networks. So there are only 126 available A class
network numbers.
- Class B IP network numbers use the leftmost 16 bits (the leftmost two
dotted quads) to identify the network, leaving 16 bits (the last two
dotted quads) to identify host interfaces. Class B addresses always have
the leftmost 2 bits of the leftmost byte set to 1 0. This leaves 14 bits
left to specify the network address giving 32767 available B class
networks. B Class networks thus have a range of 128 to 191 for the first
of the dotted quads, with each network containing up to 32,766 possible
interfaces.
- Class C IP network numbers use the leftmost 24 bits (the leftmost
three bytes) to identify the network, leaving 8 bits (the rightmost
byte) to identify host interfaces. Class C addresses always start with
the leftmost 3 bits set to 1 1 0 or a range of 192 to 255 for the
leftmost dotted quad. There are thus 4,194,303 available C class network
numbers, each containing 254 interfaces. (C Class networks with the
first byte greater than 223 are however reserved and unavailable for use).
In summary:
Network class Usable range of first byte values (decimal)
A 1 to 126
B 128 to 191
C 192 to 254
There are also special addresses that are reserved for 'unconnected'
networks - that is networks that use IP but are not connected to the
Internet, These addresses are:-
- One A Class Network
10.0.0.0
- 16 B Class Networks
172.16.0.0 - 172.31.0.0
- 256 C Class Networks
192.168.0.0 - 192.168.255.0
You will note that this document uses these sequences throughout to avoid
confusion with 'real' networks and hosts.
IP numbers can have three possible meanings:-
- the address of an IP network (a group of IP devices sharing common
access to a transmission medium - such as all being on the same Ethernet
segment). A network number will always have the interface (host) bits of
the address space set to 0 (unless the network is sub-networked - as we
shall see);
- the broadcast address of an IP network (the address used to 'talk',
simultaneously, to all devices in an IP network). Broadcast
addresses for a network always have the interface (host) bits of the the
address space set to 1 (unless the network is sub-networked - again, as
we shall see).
- the address of an interface (such as an Ethernet card or PPP interface
on a host, router, print server etc). These addresses can have any value
in the host bits except all zero or all 1 - because with the host bits all
0, the address is a network address and with the host bits all 1 the
address is the broadcast address.
In summary and to clarify things
For an A class network...
(one byte of network address space followed by three bytes of host
address space)
10.0.0.0 is an A Class network number because all the host
bits of the address space are 0
10.0.1.0 is a host address on this network
10.255.255.255.255 is the broadcast address of this network
because all the host bits of the address space are 1
For a B class network...
(two bytes of network address space followed by two bytes of host
address space)
172.17.0.0 is a B Class network number
172.17.0.1 is a host address on this network
172.17.255.255 is the network broadcast address
For a C Class network...
(three bytes of network address space followed by one byte of host
address space)
192.168.3.0 is a C Class network number
192.168.3.42 is a host address on this network
192.168.3.255 is the network broadcast address
Almost all IP network numbers remaining available for allocation at
present are C Class addresses.
The network mask is more properly called the subnetwork mask. However,
it is generally referred to as the network mask.
It is the network mask and its implications on how IP addresses are
interpreted locally on an IP network segment that concerns us most
here, as this determines what (if any) sub-networking occurs.
The standard (sub-) network mask is all the network bits in an address
set to '1' and all the host bits set to '0'. This means that the
standard network masks for the three classes of networks are:-
- A Class network mask: 255.0.0.0
- B Class network mask: 255.255.0.0
- C Class network mask: 255.255.255.0
There are two important things to remember about the network mask:-
- The network mask affects only the local interpretation of
local IP numbers (where local means on this particular network segment);
- The network mask is not an IP number - it is used to modify
how local IP numbers are interpreted locally.
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