Subnet
Addressing______________________
Subnet
Addressing
Till now we have learnt that
the IP Address of all hosts connected to the Internet are divided into two
parts-:
The
number of octets or bits allocated to the netid and the hostid parts depends on
the class or the range to which the IP Address belongs. For Example, If
an IP Address belongs to Class A, then the first 8 bits are allocated to the
netid part, while the remaining 24 bits are allocated to the hostid
part.
However, with the
introduction of Subnet Addressing, a new division is introduced which divides an
IP Address into 3 parts-:
The
introduction of Subnet Addressing not only hides the organizational structure of
the internal network, but also prevents the wastage of IP Addresses. Typically,
an IP Address belonging to Class B will be divided in the following
manner:
First 16 bits -----Network ID
Last 16 bits -----Host ID
Such a division requires
216 -2 hosts to be
attached to that particular network. However, such a large number of hosts are
seldom attached to the same network. Thus the practice of not using Subnet
addressing leads to the wastage of IP Addresses.
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HACKING FACTS:: Please note that we
subtract the 2 because a system with a Host ID of 0 or 255 is invalid i.e. a
host ID value of 0 or 255 cannot be allocated to a system, as both are reserved
for special usage. For Example, a host ID value of 0 is usually used for
routers.
********************
But, after subnetting it the IP Address would typically
look as follows:
First 16 bits -------Network ID
Next 8 bits----------Subnet ID
Next 8 bits----------Host ID
This allows for the usage of
254 hosts per subnet of the 254 subnets possible.
NOTE: I have used 8 bits for
the Subnet ID in this example. However, we can easily allocate as many bits as
we want to, to the Subnet ID part.
A
netmask value is a 32-bit value containing one bits (255’s) for the network ID
and zero bits (0’s) for the host ID. Using the netmask value one can easily
determine as to how many bits are reserved for the net ID and how many bits for
the host ID. In other words, thus, we can also say that by studying the netmask
value of an IP Address we can determine the Class to which an IP Address
belongs.
One
can find out the netmask value of an IP Address by giving the following
command:
C:\WINDOWS>route
PRINT
Active
Routes:
Network Address
Netmask
Gateway Address
Interface
Metric
127.0.0.0
255.0.0.0
127.0.0.1
127.0.0.1
1
203.94.53.12
255.255.0.0
203.94.0.0
0.0.0.0 1
202.21.87.43
255.255.255.0
202.21.87.0
0.0.0.0 1
Let
us examine the output line by line. The first line says:
Network Address
Netmask
Gateway Address
Interface
Metric
127.0.0.0
255.0.0.0
127.0.0.1
127.0.0.1
1
In
this case the netmask has a value of 255.0.0.0, which means that the first octet
contains all one bits (255’s), while the last three octets contain all zero bits
(0’s). In other words, it signifies that the first octet is the network ID (as
it contains only one bits) while the last three octets are reserved for the host
ID (as it contains only zero bits). Hence, the IP Address 127.0.0.1 is a Class A
Internet Protocol Address with 127 being the netid and .0.0.0 being the host
ID.
Coming to the second line,
we have a netmask value of 255.555.0.0, which means that the first two octets
are the network ID parts (i.e. 203.94) while the last two octets are the host ID
parts (i.e. 53.12). It also tells us that the IP Address belong to Class B of
addresses. Similarly, in the last
case the netmask value of 255.255.255.0 means that the address 202.21.87.43
belongs to Class C of addresses with the network ID being 202.21.87 and the host
ID being 43.
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HACKING FACTS: If your prime aim is to find the Class
of addresses to which an IP belongs, you need not follow the above process. By
simply knowing an IP Address and comparing it with the Class-Range Chart below,
we can easily determine the Class to which it belongs:
Class
Range
A
0.0.0.0 to 127.255.255.255
B
128.0.0.0 to 191.255.255.255
C
192.0.0.0 to 223.255.255.255
D
224.0.0.0 to 239.255.255.255
E
240.0.0.0 to 247.255.255.255
To
recapitulate we can say that the prime usage of netmask is to determine the
Class of addresses to which an IP Address belongs.
Just like Netmask, Subnet Mask too is a 32-bit value
containing one bits (255’s) for network ID and subnet ID while zero bits (0’s)
for the host ID. Subnet Mask when together used with Netmask can be used to
determine exactly how many bits are allocated for the Network ID, Host ID and
Subnet ID.
Let us take an example to make this clearer. Assume the
following data for this exercise:
IP Address: 202.12.34.77
Netmask: 255.255.0.0
Subnet Mask: 255.255.255.0
According to the netmask value, the first two octets are
reserved for usage by the network ID, while the last two octets are reserved for
host ID. Thus, we can now break down 202.12.34.77 into:
Net ID: 202.12
Host ID: 34.77
This particular IP Address has a subnet mask of
255.255.255.0, which means that the first three octets are to be used for Net ID
and the Subnet ID, while the last octet by the Host ID. Using this information,
we can break down 202.12.34.77 into:
Net ID and Subnet ID: 202.12.34
Host ID: 77
However, earlier we determined that the net ID is 202.12,
thus combining all the above information we can finally divide 202.12.34.77
into:
Net ID: 202.12
Subnet ID: 34
Host ID: 77
To recapitulate, we can say that Netmask gives us the
boundary between Host ID and Network ID, while Subnet Mask gives us the boundary
between Network ID and Subnet ID.
Whenever you connect to your ISP, you are allocated a
static IP Address (almost always), which changes each time you reconnect. In
order to determine your IP Address, Class of Addressing, host ID, network ID and
Subnet ID, simply follow the below process:
NOTE: I carried out this example on my system.
Firstly, in order to find the IP Address of my system, I
first connect to my ISP and use the
Netstat command:
C:\WINDOWS>netstat
-n
Active
Connections
Proto Local Address
Foreign Address
State
TCP 203.94.253.183:1025 64.4.13.56:1863
ESTABLISHED
TCP 203.94.253.183:1031 209.143.242.119:80 ESTABLISHED
This gives me my Dynamic IP
Address, which is, 203.94.253.183. Using the Class-Range chart I deduce that my
IP Address is using Class B addressing, which would mean that my netmask is
probably 255.255.0.0.
All
that I need to know now, in my quest to break apart my IP Address, is my Subnet
mask. In order to find the Subnet Mask of a Windows system, one need to follow
the below process:
When I tried this out on my
system, it gave me a Subnet Mask of 255.255.255.0. Combining this value with a
Netmask value of 255.255.0.0, we come to the following
information:
IP
Address: 203.94.253.183
Class of Addressing: Class
B
Network ID:
203.94
Subnet ID:
253
Host ID: 183