IP addresses and Subnetting
- IP addresses & subnetting - an overview
- IP addresses
- What is an IP address?
- Classes of IP addresses
- Globally routable and private network IP addresses
- Subnetting
- What is subnetting?
- How does subnetting work?
- Subnet masks
- Calculating a network number using a subnet mask
- Calculating a broadcast address using a subnet mask
- Prefix length notation (CIDR notation)
- Calculating a subnet mask
- Defining subnet numbers
- The fast track to the advantages of subnetting
- List of subnet masks
Test your knowledge
IP addresses & subnetting - an overview
The following gives an introduction to IP addresses and subnetting on local
area networks. If you want to find out about the advantages of using private
network IP addresses on you local area network, or what subnetting can do for
you, the explanation is here. You can also find the recipe for how you calculate
a subnet mask, a network address and broadcast address. However, the course
also offers a fast track to getting the advantages of subnetting on local area
networks without having to do all the calculations yourself. If this is what
you are looking for, you might want to jump directly to the last chapter in
this course: ‘The fast track to the advantages of subnetting’.
IP addresses
Each computer on a TCP/IP based network (including the Internet) has a unique,
numeric address called an IP address (IP stands for Internet Protocol), enabling
data packages to be addressed to this specific recipient.
What is an IP address?
An IP address consists of four so-called octets separated by dots. The octet
is a binary number of eight digits, which equals the decimal numbers from
0 to 255. To make IP addresses more easy to read and write, they are often
expressed as four decimal numbers, each separated by a dot. This format is
called "dotted-decimal notation".
An IP address in its binary and dotted-decimal notation
In a local area network based on TCP/IP, an IP address must be assigned to
each host (computer or device) in the network. The IP address must be unique
to each host. (If two hosts were given the same address, the data to these
hosts would be picked up randomly by one of them – be it the intended receiver
or not – causing network irregularities.)
In addition, a device that serves as router to another network, contains
two or more network adaptors and belongs to two or more networks. In this
case, each adaptor must be assigned a unique IP address on each network.
Part of an IP address designates the network, while another part designates
the individual host. The network number field is also referred to as the ‘network
prefix’.
The two parts of an IP address
Exactly where the network part ends and the host part begins is calculated
by routers, using a so-called subnet mask as a deciphering key.
All hosts on a given network share the same network number, but each of them must have a unique host number:
The host portion of the IP address is unique to each host
The network portion of an IP address is inherited down through a network
hierarchy, as illustrated below.
Each cloud symbolizes a network segment
Classes of IP addresses
In order to provide the flexibility required to support differently sized
networks, IP addresses come in three classes, A, B, and C. Every class
fixes the boundary between the network portion and the host portion of the
IP address at a different point. This makes them appropriate for different
size networks.
Class C addresses allow 254 hosts per network and are typically used by smaller
and middle-sized companies. Class B networks allow a maximum of 16,384 hosts,
while Class A networks allow more than 16 million hosts. As a consequence,
Class A networks are only used by really large organisations.
Calculating the number of possible hosts requires a closer look at the IP
classes in their binary form. (The binary system is a base-2 number system,
just like the base-10 number system is known as the decimal number system).
It is done as follows:
- In a Class C network only the last octet is used to designate the hosts. The maximum decimal number that you can write using eight bits is 256 (28).
The host calculation now requires that 2 is subtracted, because two host
addresses must be reserved for a network address and a broadcast address
(for a further explanation of network and broadcast addresses, see the section
on ‘Subnets’). Ergo the maximum number of hosts on a Class C network is
256-2=254.
- A class B network allows a maximum of 16,384 hosts (216-2)
per network (two octets designate the hosts).
- A class A network allows up to 16,777,214 (224-2) hosts per
network (three octets are used to designate the hosts).
The table below shows the range of dotted-decimal values that can be assigned
to each of the three address classes. An x represents the host number field
of the address which is assigned by the network administrator.
Class A networks are also referred to as ‘/8’s (pronounced slash eight’s or
just eight’s) since they have an 8-bit network prefix (one octet is used to
designate the network). Following the same convention, Class B networks are
called ‘/16s’ and Class C networks ‘/24s’.
Globally routable and private network IP addresses
There are two types of IP addresses – those which are globally routable
(included in the routing tables on the Internet), and those which have been
set aside for private networks. It is generally recommended that organisations
use IP addresses from the blocks of private network addresses for hosts that
require IP connectivity within their company network, but do not require external
connections to the global Internet.
The system with non-routable IP addresses was introduced to help prevent
a future shortage of IP addresses due to the explosive growth of the Internet.
Because addresses belonging to these address blocks are not routed through
the Internet routing system, the same numbers can be used at the same time
by many different organisations.
The three blocks of IP addresses which have been reserved for private networks are:
There are no official rules for when to use which of the three private network
IP address blocks, but generally the one of the most suitable size is used.
For obvious reasons there is no need to use 10.x.x.x if it is unthinkable
that your LAN will ever grow to more than 254 hosts. However, when using private
addresses the network administrator can be liberal on the usage of the addresses
when assigning them to the different parts of a network, as the strict rules
that govern public IP address assignment do not apply.
Hosts with private network IP addresses cannot communicate directly with
the Internet, because the Internet refuses to receive and transmit data with
such origin or destination address. For a host with a private network IP address
to be allowed to communicate with the Internet, it must have its data stream
to the Internet handled by an intermediary host, which can act as an ‘Internet
representative’ for the private host. The intermediary host must have ways
to relay data between the global Internet and the host on the private network.
Therefore it must have a globally routable IP address that it uses when communicating
with the Internet, and a private network IP address that is uses for communication
with the private host. There are a number of different types of intermediary
hosts that fit this description. The most common types of intermediary hosts
are proxy servers, firewalls and firewalls with NAT (Network Address Translation).
A NAT router translating private network IP addresses to globally routable IP addresses
An advantage of using private network addresses is that it makes it easier
for organisations to change their Internet service provider without having
to renumber their IP addresses. If private network addresses are not used,
renumbering when changing ISP is necessary because globally routable IP addresses
are "owned" by the Internet service provider that the company has
"leased" the IP addresses from. It is possible to buy and own IP
addresses, but this only applies to very large organisations that need in
the magnitude of 40,000 globally routable IP addresses.
Using private network IP addresses also gives a company a measure of security.
Globally routable IP addresses are advertised in the routing tables on the
Internet, making the system vulnerable to hackers. When private IP network
addresses are used, however, the intermediary host (such as a firewall with
NAT) will work as a barrier against unwanted visits from the Internet.
The current version of IP, IP version 4, defines a 32-bit address, which
means that there are only 232 (4,294,967,296) addresses available
globally. Over the past few years, the number of available IP addresses on
the Internet has started to run out, as the number of companies and people
wishing to go on-line has exploded. As a consequence, a new generation of
IP addresses (IPv6) is currently in the works. The current IP system will
not become obsolete overnight, however, as the two systems will coexist for
some time after the new version has been implemented.
Subnetting
What is subnetting?
A subnet is a segment of a network. Subnetting is a technique that allows
a network administrator to divide one physical network into smaller logical
networks and, thus, control the flow of traffic for security or efficiency
reasons.
Dividing a network into several subnets can serve a number of purposes: to
reduce network traffic by decreasing the number of broadcasts (if used in
combination with a switch), to exceed the limitations in a local area network,
for instance the maximum number of allowed hosts, or to enable employees to
be able to dial in to the network from home, without opening the entire network
up to unwanted visits from the Internet.
Subnets are created by using a so-called subnet mask to divide a single Class
A, B, or C network number into smaller pieces, thus allowing an organisation
to add subnets without having to obtain a new network number through an Internet
service provider. Subnets can again be subnetted into sub-subnets.
Subnets were originally invented to help solve the lack of IP addresses on
the Internet.
Please note: There is a fast track to getting the advantages of subnetting
on local area networks without having to go through the process of calculating
a subnet mask, etc. The recipe can be found in the last chapter: ‘The
fast track to the advantages of subnetting’.
How does subnetting work?
An IP address consists of a network portion and a host portion. A subnet
is created by borrowing bits from the part of the IP address which normally
designates the host and using them to designate one or more smaller, secondary
networks (subnets) within the original network. The network prefix and subnet
number in combination are called the extended network prefix (in every day
talk often, somewhat confusingly, referred to as the network number!).
Subnet masks
A 32-bit subnet mask is used as a deciphering key to determine how an IP
address is to be divided into extended network prefix and host part. It is
used by routers and network devices to determine where traffic should be routed
to.
Like IP addresses, subnet masks consist of four numbers of 8 bits, separated
by dots. They are usually written in the corresponding decimal notation.
The typical subnet masks used for Class A, B and C addresses are as follows:
All the 0’s in the subnet mask specify that this part in a corresponding
IP address is the host portion, while the 1’s indicate that the corresponding
bits in the IP address constitute the network portion.
The three subnet masks above set the change from network to host portion
at the end of a whole octet – Class A after one octet, Class B after two octets,
and Class C after three. However, a subnet masks does not have to follow the
address classes, but can specifiy a host portion that is not a whole octet.
The subnet mask 255.255.255.240 (11111111.11111111.11111111.11110000)
for instance, marks the breaking point four bits into the last octet.
The purpose of having subnet masks defining networks is that the technical
devices that the network is build from will be able to determine if traffic
should be routed out of the network or kept within it. Using a mask saves
the routers from having to handle the entire 32-bit address, because they
can simply look at the bits selected by the mask (and thus not worry about
the host portion of the address).
Internet routers use only the network number of the destination address to
route traffic to a subnetted environment. Subnetting, thus, also has the advantage
that it keeps the size of the routing tables on the Internet down because
Internet routers only need to know the one common network address for all
the individual computers and devices on the different subnets. The route from
the Internet to any subnet of a network is the same, no matter which subnet
the destination host is on, namely that of the mother network. From there,
the local network router(s) divides the communication out into individual
subnets and to the individual hosts on these subnets.
Subnetting keeps the size of the routing tables on the Internet down, as the Internet routers only use the network number of a subnetted environment to route traffic to any of the subnets
A router within a subnetted environment uses the extended network prefix to
route traffic between the individual subnets. The extended network prefix is
composed of the network prefix and the subnet number.
Calculating a network number using a subnet mask
The network number is the part of the IP address that all hosts on a network
share. Network numbers are entered in routing tables and used by routers to
direct traffic between networks. The network number, or extended network
prefix, of an IP address is found by using the subnet mask to mask off
the host portion of the IP address.
An example:
You choose the IP address 192.168.1.1 and the subnet mask 255.255.255.0.
The above IP address and subnet mask written in their binary notation looks
as follows:
Every bit in the IP address is compared to the corresponding bit in the subnet
mask: a ‘1’ in the subnet mask indicates that the corresponding bit in the
IP address is part of the network portion, while a ‘0’ in the subnet mask
illustrates that the corresponding bit in the IP address is part of the host
portion.
In the above example, the host portion is thus all the bits in the first
three octets, which in decimal numbers is written 192.168.1.0.
Subnet masks written in binary notation always consist of a continuous string
of 1’s followed by a continuous string of 0’s, e.g.
As a consequence, the host range that a subnet mask defines will always be
either 2 (21 – corresponding to a situation where only the last
bit defines hosts), 4(22 – corresponding to a situation where the
last two bits define hosts), 8 (23), 16(24), 32(25),
64(26), 128(27) or 256(28).
In reality, 2 must be subtracted from all the numbers of hosts above to get
the actual number of IP addresses available to use for hosts, because two
addresses, namely the address which has all-0’s in the host bits (this network)
and the address which has all 1’s in the host bits (broadcast), can not be
assigned to hosts. As a consequence, it is not possible to make a network
that consists of fewer than four IP addresses (2 hosts + the broadcast and
network addresses).
In the above example, based on the IP address 192.168.1.1 and the subnet
mask 255.255.255.0, the network address (all host bits set to 0) was 192.168.1.0.
The broadcast address for this network would be 192.168.1.255 as illustrated
below.
Calculating a broadcast address using a subnet mask
The broadcast address is the address where all the bits in the host portion
are set to 1. The broadcast address is used when you want to communicate data
to all the hosts on a network. Here follows an example of how it can be calculated:
In our example above, the last 8 bits were hosts. As a consequence, the broadcast
address for the network 192.168.1.0 with the subnet mask 255.255.255.0 is
11000000.10101000.00000001.111111111 (host bits set to 1) or in decimal
notation: 192.168.1.255
Note: If you know the IP address segment your network consists off, the lowest
IP address is the network number, while the highest IP address is the broadcast
address.
Prefix length notation (CIDR notation)
For the sake of convenience, prefix length notations (CIDR notation, Classless
Inter-Domain Routing notation) is often used instead of writing the subnet
mask. This means that the IP address above (192.168.1.1) with the subnet mask
255.255.255.0 can also be expressed as 192.168.1.1/24. The /24 indicates the
network prefix length, which is equal to the number of continuous one-bits
in the subnet mask.
Calculating a subnet mask
When subnetting a network, you first need to determine two things:
- how many subnets do you need to create?
- how many host addresses do you need on each net (you should always add
some extra host addresses to be used for future growth).
Once you have determined the required number of subnets and hosts, the next
step is to calculate a corresponding subnet mask, which will support the desired
network structure.
In the following you will find two examples of how the subnetting of a Class
C network can be planned and the required subnet mask calculated.
Example A:
Imagine that you are setting up a network on the network number 192.168.1.0/24.
You need a local area network which is going to connect a number of workstations,
servers and others devices, totalling more than 80. To allow some slack, you
set the number of required hosts to be 90. Now, the calculation of the subnet
mask can begin. The calculation is best understood if the numbers are looked
at in their binary form (see example below).
The first step is to determine the lowest number of bits required to identify
90 hosts. Since IP addresses of hosts can only be created along binary boundaries,
the number of hosts must be created in blocks of powers of two – 2 (21),
4 (22), 8 (23), 16 (24) and so on. In other
words, we must first determine what the lowest power is that we can lift 2
to and get a number equal to or greater than 90. Since 27 equals
128 and 26 equals 64, we need 7 bits to designate 90 hosts. This
means that the host portion of the IP address must be the last 7 bits. An
IP address consists of 32 bits all in all. The network portion must thus consist
of 32-7=25 bits. As every ‘1’ in a subnet mask indicates that the corresponding
bit in the IP address belongs to the network portion and every ‘0’ indicates
that the corresponding bit in the IP address is part of the host portion,
the corresponding subnet mask must consist of a series of 25 1’s, followed
by 7 0’s (as illustrated below). Written in decimal notation, the subnet mask
is 255.255.255.128.
The number of subnets that can be created using this subnet mask is calculated
as follows: The original network prefix was 24 bit long (192.168.1.0/24),
and the extended network prefix (network prefix + subnet prefix) masked off
by the subnet mask is 25 bits long. As a consequence, one bit is available
to designate subnets. In other words, it is possible to create 2 (21)
subnets of this given size using this subnet mask, should we wish to do so.
Example B:
Now pretend that through an estimation of the number of subnets and hosts
that the subnet you are setting up will have to support, you have come to
the conclusion that you need to define ten subnets. The largest subnet is
required to support 10 hosts. You have again chosen to create the subnet on
the network number 192.168.1.0/24. Now, the calculation of the subnet mask
can begin.
The first step is to determine the number of bits required to define the
ten subnets. Since a network address can be subnetted only along binary boundaries,
subnets must be created in blocks of powers of two 2 – 2, 4, 8, 16 and so
on. Thus, it is impossible to define an IP address block so that it contains
exactly ten subnets. In this case, the network administrator must define a
block of 16 (24) and have six unused subnet addresses for future
growth.
Since we need to raise 2 to the power of four (24) to get 16,
four bits are required to designate the sixteen subnets in the block. In this
example, you are subnetting a Class C network (/24) so it will need four more
bits (/28) as the extended network prefix. A 28-bit extended network prefix
can be expressed in dotted-decimal notation as 255.255.255.240. This is illustrated
below:
A 28-bit extended network prefix leaves 4 bits to define host addresses on
each subnet. This means that each subnet with a 28-bit prefix represents a
continuous block of 24 (16) individual IP addresses. However, since
the all-0’s (‘this network’) and the all-1’s host addresses (‘broadcast’)
must not be allocated, there are 14 (24-2) assignable host addresses
on each subnet. We needed a maximum of 10 hosts on each subnet, so the result
is satisfactory.
Defining subnet numbers
In example B above, with the ten subnets, the subnets will be numbered 0
through to 9. The 4-bit binary representation of the decimal values 0 though
9 are: 0 (0000), 1 (0001), 2 (0010), 3 (0011), 4 (0100), 5 (0101), 6 (0110),
7 (0111), 8 (1000), 9 (1001), 10 (1010).
To find the subnet number of each subnet, place the binary representation
of the subnet number, e.g. 0001, into the bits in the base network address
that is used to designate the subnet (see illustration below). For example,
to define subnet number 8, the network administrator places the binary representation
of 8 (1000) into the 4-bits in the base network address that are used to designate
the subnet.
The ten subnet numbers for the example are given below. The italicised portion
of each address identifies the extended network prefix, while the bold digits
identify the 4 bits representing the bits in the address that are used to
designate the subnet:
An easy way to ensure that the subnets are calculated correctly is to ensure
that they are all multiples of the subnet number 1 address. In this case,
all subnets are multiples of 16.
The fast track to the advantages of subnetting
There is a fast track to getting the advantages of subnetting on local area
networks without having to go through the process of calculating a subnet
mask, etc. The fast track involves using a standard class subnet mask in combination
with addresses from the IP address blocks set aside for private networks.
For instance by using 192.168.0.0 to designate your local network 1, 192.168.1.0
to designate your local area network 2 and 192.168.2.0 to designate your local
area network 3. The standard subnet mask to use on networks with up to 254
hosts is 255.255.255.0.
The three blocks of IP addresses which have been reserved for private networks and the corresponding standard subnet masks are:
If you do not want to use this method, a list of precalculated subnet masks
together with the number of hosts available on the networks they create can
be found below for easy reference.
List of subnet masks
The list below can be used as a fast track when subnetting. It describes
the relationship between the number of host IP addresses required and the
corresponding subnet mask. The example above, for instance, required subnets
with 10 host addresses on each. The nearest number that is equal to or greater
than 10 is 16. The subnet mask corresponding to 16 hosts is listed in the
table below. It is 255.255.255.240.
|
