| The issue
of subnetting has long been a mystery to many system
administrators. It seems that there are just too many
bits, bytes, and masks involved to make it worthwhile.
Besides, who needs all that trouble when it's all you
can do to just maintain the network as it is? You can
certainly find sympathy, but if your network plans call
for Internet access then IP (Internet Protocol)
addressing and possibly subnet masking are topics that
you s hould be familiar with.
As networks grow to
increasing numbers of segments, more network address
numbers will be needed as each segment requires an
individual number. The InterNIC (whose name signifies
cooperation of Network Information Centers or NICs),
however, is not eager to hand out unlimited network
addresses because they are quickly running out of them.
The Internet community is generally taking a firm stand
on limiting address availability, due to the accelerated
growth demand for Internet access. Network
administrators will have to work with what they have to
better accommodate network requirements and the
shrinking supply of address numbers.
One way of accomplishing
this address conservation is to take the address that is
assigned to your network and expand the capacity with
subnets. Subnetting allows you to increase the number of
networks available to you without applying for another
IP address number.
IP Addressing
The IP addressing
framework allows one to address about 16,000,000 unique
hosts for a Class A address, around 65,000 hosts for a
Class B address, but only 254 hosts for a Class C
address. However, there are no more Class A addresses
available, and the InterNIC has stopped assigning Class
B addresses. Class C addresses are the most numerous,
but their limitation is that each can identify only 254
unique hosts.
The IP address is
composed of 32 bits, which consist of two parts: the
most significant bits (MSBs) identify a particular
network and the remaining bits specify a host on that
network. The most significant bits of the network
portion actually determine the address class as shown in
this table:
Address MSB
Class Pattern
Class A 0
Class B 10
Class C 110
Class A Address Example
A class A address could
be diagramed:
Network Host
+------+ +----------------------+
| | | |
[0xxxxxxx][xxxxxxxxxxxxxxxxxxxxxxxx]
which shows the eight
network bits followed by the 24 host bits.
These 32-bit IP
addresses are almost always written as four
dot-separated decimal numbers, one for each byte of the
address. Thus, our class A address would have a range of
address numbers from 1.0.0.0 through 126.0.0.0 ( 0.
x.x.x and 127. x.x.x are reserved). The number of host
addresses per network is 16,777,214, which is two less
than two raised to the 24th power because both host
numbers 0.0.0 and 255.255.255 are reserved.
In practice, people
don't really attach 16 million hosts to a network so
administrators of a Class A site often divide the host
address portion into a (sub)network and host portion. (Subnetting
is now supported by most operating systems.) Each Class
A network number can support up to 65,534 subnets
(network numbers 0.0 and 255.255 are reserved) with each
having 254 hos ts (host numbers 0 and 255 are reserved).
This is done by using the 16 high -order bits of the
host portion for the subnet number and the lower eight
bits for the host as diagramed here:
Network Subnet Host
+------+ +--------------+ +------+
| | | | | |
[0xxxxxxx][xxxxxxxxxxxxxxxx][xxxxxxxx]
Class B Addresses
The first two bits of a
Class B address are 1 and 0, the next fourteen bits
identify the network and the last sixteen the host, as
diagramed:
Network Host
+--------------+ +--------------+
| | | |
[10xxxxxxxxxxxxxx][xxxxxxxxxxxxxxxx]
Thus, Class B addresses
include the network numbers in the range from 128.1.0.0
through 191.254.0.0 for a total of 65,534 host
addresses.
As with the Class A
address, we can divide the host portion of a Class B
address into subnet and host parts. For instance, let's
spli t our Class B network number on the byte boundary,
that is, the eight MSBs of the host portion identifies
the subnet and the remaining bits the host, as
diagramed:
Network Subnet Host
+--------------+ +------+ +------+
| | | | | |
[10xxxxxxxxxxxxxx][xxxxxxxx][xxxxxxxx]
This arrangement allows
254 subnets each with 254 hosts.
Other Address Classes
The first three bits of
a Class C address are 1, 1, and 0, the next 21 bits
identify the network and the last eight the host, as
diagramed:
Network Host
+----------------------+ +------+
| | | |
[110xxxxxxxxxxxxxxxxxxxxx][xxxxxxxx]
Thus, Class C addresses
include the network numbers in the range 192.0.1.0
through 223.255.254.0 for a total of 254 host addresses
per network address.
Finally, we have Class D
and Class E addresses. Class D address start at
224.0.0.0 and are used for multicast purposes. Class E
addresses start at 240.0.0.0 and are currently used only
for experimental purposes.
The Subnet Mask
A subnet mask (or
number) is used to determine the number of bits used for
the subnet and host portions of the address. The mask is
a 32-bit value that uses one-bits for the network and
subnet portions and zero-bits for the host portion.
Let's look at an
example. Here we have a Class B address of 191.70.55.130
and apply some different subnet masks. A logical AND
operation is performed between the IP address and the
subnet mask as shown:
Here we use a mask that
retains the default 16 network and host bits for a Class
B address:
191 70 55 130
1011 1111 1000 0110 0011 0111 1000 0010 IP address
1111 1111 1111 1111 0000 0000 0000 0000 Subnet mask
1011 1111 1000 0110 0000 0000 0000 0000 Result
Here we employ a mask
that d ivides the host portion into a subnet and host
that are each eight bits wide:
191 70 55 130
1011 1111 1000 0110 0011 0111 1000 0010 IP address
1111 1111 1111 1111 1111 1111 0000 0000 Subnet mask
1011 1111 1000 0110 0011 0111 0000 0000 Result
This division allows 254
(256-2 reserved) subnets, each with 254 hosts.
This division on a byte
boundary makes it easy to determine the subnet and host
from the dotted-decimal IP address. However, the
subnet-host boundary can be at any bit position in the
host portion of the IP address. Here, we use a mask that
allows more subnets (512-2 reserved), but with the
trade-off of fewer hosts (128-2) per subnet:
191 70 55 130
1011 1111 1000 0110 0011 0111 1000 0010 IP address
1111 1111 1111 1111 1111 1111 1000 0000 Subnet mask
1011 1111 1000 0110 0011 0111 1000 0000 Result
The subnet-host number t radeoff
Here's a table that
let's you see at a glance the trade off between the
number of subnets and hosts with different subnet masks
for both Class B and Class C addresses. We've already
subtracted two from the results in the last two columns
to take the reserved network and host numbers into
account:
Class B Subnetting:
# Mask Bits Subnet Mask # Subnets # Hosts
2 255.255.192.0 2 16382
3 255.255.224.0 6 8190
4 255.255.240.0 14 4094
5 255.255.248.0 30 2046
6 255.255.252.0 62 1022
7 255.255.254.0 126 510
8 255.255.255.0 254 254
9 255.255.255.128 510 126
10 255.255.255.192 1022 62
11 255.255.255.224 2046 30
12 255.255.255.240 4094 14
13 255.255.255.248 8190 6
14 255.255.255.252 16382 2
Class C Subnetting:
# Mask Bits Subnet Mask # Subnets # Hosts
2 255.255.255.192 2 62
3 255.255.255.224 6 30
4 255.255.255.240 14 14
5 255.255.255.248 30 6
6 255.255.255.252 62 2
The Subnet Advantage
Subnetting hides the
internal network organization to external routers and
thus simplies routing. For instance, a subnetted Class B
address would require fewer routes than the equivalent
number of Class C addresses. Shorter routing tables mean
faster network transfers.
Subnetting allows
address administration to be decentralized. Besides
technical advantages, this approach may also provide
political benefits for the organization. For instance,
an administrator could assi gn a subnet to a department,
which would then be responsible for their own network
management.
Subnetting can help
overcome distance limitations of physical networks by
dividing up a physical network into individually
addressed networks so they can be connected logically
with routers.
Example: Subnetting a Class C Network
One of the first things
a network administrator needs to do is define the
requirements for the network. The logical place to start
is to consider how many hosts are on the network.
Using the maximum number
of hosts on one Ethernet segment is generally not good
practice because it could create performance problems
due to network congestion. If you only have one Class C
address assigned to your network then what can you do?
Refer to our table above that depicts the Class C
address subnetting network number-host trade off.
Even though a Class C
address can support up to 254 hosts, in my experience,
60-80 hosts is a good number for most LANs using of fice
automation tools. I've seen overloaded Ethernet
segments--with over 100 hosts--at client sites. My
recommendation is that they segment their LAN in half or
even further. Also, many hub cards come with 24 ports
per card, which makes it easy to segment in 24-host
multiples provided that the hub supports multiple
segments on the backplane. Many do.
One reasonable approach
would be to select six subnets each with 30 hosts.
Although two subnets with 62 hosts is also feasible, it
is not as flexible because there are only two subnets.
The other alternatives that use more subnets probably
don't provide enough hosts per subnet.
Subnets 0 and 7 are
unusable because they are used for special addressing
situations. For instance, a subnet of 7 (all one bits)
is reserved for an all subnets-directed broadcast (a
broadcast sent to all subnets of the specified subnetted
network) when the host bits are all one. This leaves
subnets 1 through 6 available for use.
In each subnet, the
first host number (0) is reserved, and the resulting
number is known as the network number. The last number
in each subnet is reserved for the broadcast address,
and cannot be used for a host address. Consequently, in
this case there are only 30 host addresses available for
each subnet. |
