IP
Addressing
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.
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:
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
A 0
Class
B 10
B 10
Class
C 110
C 110
Class
A Address Example
A Address Example
A class A address could be
diagramed:
diagramed:
Network Host
+——+
+———————-+
+———————-+
|
| | |
| | |
[0xxxxxxx][xxxxxxxxxxxxxxxxxxxx
xxxx]
which shows the eight network bits
followed by the 24 host 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.
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:
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
Subnet Host
+——+
+————–+ +——+
+————–+ +——+
|
| | |
| |
| | |
| |
[0xxxxxxx][xxxxxxxxxxxxxxxx][xxxxxxxx]
Class
B Addresses
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:
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.
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:
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.
each with 254 hosts.
Other
Address Classes
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:
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.
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.
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
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.
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:
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:
default 16 network and host bits for a Class B address:
191
70 55 130
70 55 130
1011
1111 1000 0110 0011 0111
1000 0010 IP address
1111 1000 0110 0011 0111
1000 0010 IP address
1111
1111 1111 1111 0000 0000
0000 0000 Subnet mask
1111 1111 1111 0000 0000
0000 0000 Subnet mask
1011
1111 1000 0110 0000 0000
0000 0000 Result
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:
the host portion into a subnet and host that are each eight bits wide:
191
70 55 130
70 55 130
1011
1111 1000 0110 0011 0111
1000 0010 IP address
1111 1000 0110 0011 0111
1000 0010 IP address
1111
1111 1111 1111 1111 1111
0000 0000 Subnet mask
1111 1111 1111 1111 1111
0000 0000 Subnet mask
1011
1111 1000 0110 0011 0111
0000 0000 Result
1111 1000 0110 0011 0111
0000 0000 Result
This division allows 254 (256-2
reserved) subnets, each with 254 hosts.
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:
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
70 55 130
1011
1111 1000 0110 0011 0111
1000 0010 IP address
1111 1000 0110 0011 0111
1000 0010 IP address
1111
1111 1111 1111 1111 1111
1000 0000 Subnet mask
1111 1111 1111 1111 1111
1000 0000 Subnet mask
1011
1111 1000 0110 0011 0111
1000 0000 Result
1111 1000 0110 0011 0111
1000 0000 Result
The
subnet-host number t radeoff
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:
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
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
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
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.
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.
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.
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
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.
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.
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.
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.
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.
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 ho st
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.
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.
