Network layer (Part III)
Basics of Subnetting : Classical IP
Addressing
• Network administrators sometimes need to
divide networks, especially large ones, into
smaller networks.
• These smaller divisions are called subnetworks
and provide addressing flexibility.
• Most of the time subnetworks are simply
referred to as subnets
Basics of Subnetting : Classical IP
Addressing
• Similar to the host number portion of Class A,
Class B, and Class C addresses, subnet
addresses are assigned locally, usually by the
network administrator.
• Also, like other IP addresses , each subnet
address is unique.
Basics of Subnetting : Subnetwork
• Subnet addresses include the Class A, Class B, or Class
C network portion, plus a subnet field and a host field.
• The subnet field and the host field are created from the
original host portion for the entire network.
• The ability to decide how to divide the original host
portion into the new subnet and host fields provides
addressing flexibility for the network administrator.
• To create a subnet address, a network administrator
borrows bits from the original host portion and
designates them as the subnet field.
Figure 1
Basics of Subnetting : Subnetwork
• Internally, networks may be divided into smaller networks
called sunetworks, or simply sub-nets.
• By providing a third level of addressing, subnets provide extra
flexibility for the network administrator.
• For example, a class “B” network provided by the interNIC,
can be broken up into many networks.
• In this example, 131.108.1.0, 131.108.2.0 and 131.108.3.0 are
all subnets within the network 131.108.0.0
• Figures 1 and 2 illustrate the hierarchical nature of subnet
addresses.
Figure 2
Basics of Subnetting : Subnetwork
• To create a subnet address, a network administrator
borrows bits from the host field and designates them as the
subnet field.
• The minimum number of bits that can be borrowed is 2.
• If you were to borrow only 1 bit, to create a subnet, then
you would only have a network number - the .0 network and the broadcast number - the .1 network.
• maximum number of bits that can be borrowed can be any
number that leaves at least 2 bits remaining, for the host
number.
• In this example of a Class C IP Address, bits from the host
field for the subnet field have been borrowed.
Basics of Subnetting : Purpose for
subnetting
• A primary reason for using subnets is to
reduce the size of a broadcast domain.
• Broadcasts are sent to all hosts on a network
or subnetwork.
• When broadcast traffic begins to consume too
much of the available bandwidth, network
administrators may choose to reduce the size
of the broadcast domain.
Basics of Subnetting : Subnet mask
• The subnet mask (formal term: extended
network prefix), tells the network devices
which part of an address is the network field
and which part is the host field.
• A subnet mask is 32 bits long and has 4
octets, just like an IP address.
Basics of Subnetting : Subnet mask
• To determine the subnet mask for a particular subnetwork
IP address follow these steps.
• (1) Express the subnetwork IP address in binary form.
• (2) Replace the network and subnet portion of the address
with all 1s.
• (3) Replace the host portion of the address with all 0s.
• (4) As the last step convert the binary expression back to
dotted-decimal notation.
• Note: The extended network prefix includes the class A, B,
or C network number, plus the subnet field (or subnet
number) that is being used to extend the routing
information (which is otherwise just the network number).
Basics of Subnetting : Boolean
operations: AND, OR, and NOT
• The term "operations" in mathematics refers to rules
that define how one number combines with other
numbers.
• Decimal number operations include addition,
subtraction, multiplication, and division.
• There are related, but different, operations for working
with binary numbers.
• The basic Boolean operations are AND, OR, and NOT.
– AND is like multiplication
– OR is like addition
– NOT changes 1 to 0, and 0 to 1
Basics of Subnetting : Performing the
AND function
• The lowest numbered address in an IP
network is the network address (the network
number plus 0 in the entire host field).
• This also applies to a subnet: the lowest
numbered address is the address of the
subnet.
Basics of Subnetting : Boolean
operations: AND, OR, and NOT
• In order to route a data packet, the router must
first determine the destination network/subnet
address by performing a logical AND using the
destination host's IP address and the subnet
mask.
• The result will be the network/subnet address.
• In the Figure, the router has received a packet for
host 131.108.2.2 - it uses the AND operation to
learn that this packet should be routed to subnet
131.108.2.0.
Creating a Subnet : Range of bits
needed to create subnets
• To create subnets, you must extend the routing
portion of the address.
• The Internet knows your network as a whole,
identified by the Class A, B, or C address, which
defines 8, 16, or 24 routing bits (the network
number).
• The subnet field will become additional routing
bits, so that the routers within your organization
can recognize different locations, or subnets,
within the whole network
Creating a Subnet : Range of bits
needed to create subnets
• Question: In the address 131.108.0.0, which are
the
routing
bits?
Answer: 131.108 - That's the 16 bit Class B
network number.
• Question: What are the other two octets (16 bits)
of the address 131.108.0.0 used for?
Answer: Well, as far as the Internet knows, that's
just a 16 bit host field, because that's what a
Class B address is - a 16 bit network number and
a 16 bit host number.
Creating a Subnet : Range of bits
needed to create subnets
• Question: What part of the address 131.108.0.0 is the
subnet
field?
Answer: When you decide to create subnets, you must
divide the original host field (16 bits in the case of Class B)
into two parts - the subnet field and the host field. This is
sometimes referred to as "borrowing" some of the original
host bits to create the subnet field. The other networks in
the Internet won't care - they look at the address the same
- all they really see is the Class A, B, or C network number,
and send the packet on to its destination. The minimum
number of bits that you can borrow is 2, regardless of
whether you're working with a Class A, B, or C network1
because at least 2 bits must remain for host numbers2, the
maximum varies by address class.
Creating a Subnet : Range of bits
needed to create subnets
Creating a Subnet : Range of bits
needed to create subnets
• The subnet field always follows immediately
after the network number.
• That is, the borrowed bits must be the first n
bits of the default host field, where n is the
desired size of the new subnet field.
• The subnet mask is the tool used by the router
to determine which bits are routing bits and
which bits are host bits
Creating a Subnet : Range of bits
needed to create subnets
• Previous standards did not allow for the use of subnets
obtained by borrowing 1 bit (with only 1 subnet bit, the
subnet field can only have two values: subnet 0 is part of
the network address, and subnet 1 would be part of the
network broadcast address) – although many devices can
now support subnets obtained by borrowing 1 bit, it is still
common practice to avoid doing this to insure compatibility
with legacy devices; for our purposes here, you will always
borrow at least 2 bits.
• Similarly, a 1 bit host field would allow only for host 0,
which is part of the network address, and host 1, which is
part of the broadcast address, leaving 0 valid host
addresses.
Creating a Subnet : Determining
subnet mask size
• Subnet masks use the same format as IP
addresses.
• They are 32 bits long and are divided into four
octets, written in dotted decimal format.
• Subnet masks contain all 1s in the network bit
positions (determined by the address class) as
well as the desired subnet bit positions, and
contain all 0s in the remaining bit positions,
designating them as the host portion of an
address.
Creating a Subnet : Determining
subnet mask size
• By default, if you borrow no bits, the subnet
mask for a Class B network would be
255.255.0.0, which is the dotted decimal
equivalent of 1s in the 16 bits corresponding
to the Class B network number.
• If 8 bits were to be borrowed for the subnet
field, the subnet mask would include 8
additional 1 bits, and would become
255.255.255.0.
Creating a Subnet : Determining
subnet mask size
• For example, if the subnet mask
255.255.255.0 were associated with the Class
B address 130.5.2.144 (8 bits borrowed for
subnetting), the router would know to route
this packet to subnet 130.5.2.0 rather than to
just network 130.5.0.0
Creating a Subnet : Determining
subnet mask size
• Another example is the Class C address 197.15.22.131,
with a subnet mask of 255.255.255.224.
• With a value of 224 in the final octet (11100000 in
binary), the 24 bit Class C network portion has been
extended by 3 bits, to make the total 27 bits.
• The 131 in the last octet now presents the third usable
host address in the subnet 197.15.22.128.
• The routers in the Internet (that don't know the subnet
mask) will only worry about routing to the Class C
network 197.15.22.0, while the routers inside that
network, knowing the subnet mask, will be looking at
27 bits to make a routing decision.
Creating a Subnet : Computing subnet
mask and IP address
• Whenever you borrow bits from the host field,
it is important to note the number
of additional subnets that are being created
each time you borrow one more bit.
• You have already learned that you cannot
borrow only 1 bit; the fewest you may borrow
is 2 bits.
Creating a Subnet : Computing subnet
mask and IP address
• Borrowing 2 bits creates four possible subnets
(22) (but you must always remember that
there are two reserved/unusable subnets).
Each time you borrow another bit from the
host field, the number of subnets created
increases by a power of 2.
Creating a Subnet : Computing subnet
mask and IP address
• The eight possible subnets that are created by
borrowing 3 bits is equal to 23 (2 x 2 x 2).
• The sixteen possible subnets created by
borrowing 4 bits is equal to 24 (2 x 2 x 2 x 2).
• From these examples, it is easy to see that
each time you borrow another bit from the
host field, the number of possible subnets
doubles.
Creating a Subnet : Computing subnet
mask and IP address
• Question: How many bits are being borrowed (how long is the
subnet field) for a Class B network using a subnet mask of
255.255.240.0?
Answer: The first two octets of the mask (255.255) correspond with
the 16 bits in a Class B network number. Remember that the subnet
field is represented by all the additional "1" bits past that. The
number 240 decimal is 11110000 in binary, and you can see that
you are using 4 bits for the subnet field.
• Question: How many possible subnets are there with a 4 bit subnet
field?
Answer: Start with finding the smallest 4 bit number - 0000 - then
the largest 4 bit number - 1111 (15). So the possible subnets are 015, or sixteen subnets. However, you know you cannot use subnet 0
(it's part of the network address), and you cannot use subnet 15
(1111) either (broadcast address). So this 4 bit subnet field gives
you fourteen usable subnets (1-14).
Creating a Subnet : Computing hosts
per subnetwork
• Each time you borrow 1 bit from a host field,
there is 1 less bit remaining in the field that
can be used for host numbers.
• Specifically, each time you borrow another bit
from the host field, the number of host
addresses that you can assign decreases by a
power of 2 (gets cut in half).
Creating a Subnet : Computing hosts
per subnetwork
• To help you understand how this works, use a Class C network
address as an example.
• If there is no subnet mask, all 8 bits in the last octet are used for the
host field.
• Therefore, there are 256 (28) possible addresses available to assign
to hosts (254 usable addresses, after you subtract the 2 you know
you can't use).
• Now, imagine that this Class C network is divided into subnets. If
you borrow 2 bits from the default 8 bit host field, the host field
decreases in size to 6 bits.
• If you write out all of the possible combinations of 0s and 1s that
could occur in the remaining 6 bits, you would discover that the
total number of possible hosts that could be assigned in each
subnet would be reduced to 64 (26).
• The number of usable host numbers would be reduced to 62.
Creating a Subnet : Computing hosts
per subnetwork
• In the same Class C network, if you borrow 3
bits, the size of the host field decreases to 5
bits and the total number of hosts that you
could assign to each subnet would be reduced
to 32 (25).
• The number of usable host numbers would be
reduced to 30.
Creating a Subnet : Computing hosts
per subnetwork
• The number of possible host addresses that can
be assigned to a subnet is related to the number
of subnets that have been created.
• In a Class C network, for example, if a subnet
mask of 255.255.255.224 has been applied, then
3 bits (224 = 11100000) would have been
borrowed from the host field.
• The useable subnets created are 6 (8 minus 2),
each having 30 (32 minus 2) useable host
addresses
Creating a Subnet : Computing hosts
per subnetwork
• Exercise:
Divide the last octet into two parts: a subnet field and a host field. If there
are 32 possible host addresses that can be assigned to each subnet, then
their IP addresses would fall within the range of numbers (but remember
the 2 unusable host addresses in each subnet!).
• In a Class C network 199.5.12.0 with subnet mask 255.255.255.224, to
which subnet would host 199.5.12.97 belong? (hint: 97 = 01100001
binary)
A. subnet 0?
B. subnet 1?
C. subnet 2?
D. subnet 3?
E. subnet 4?
F. none of the above?
Creating a Subnet : Boolean AND
operation
• As you have already learned, the lowest
numbered address in an IP network is the
network address (the network number plus 0
in the entire host field).
• This also applies to a subnet; the lowest
numbered address is the address of the
subnet
Creating a Subnet : Boolean AND
operation
• In order to route a data packet, the router must first
determine the destination network/subnet address.
• To accomplish this the router performs a logical AND
using the destination host's IP address and the subnet
mask for that network.
• Imagine that you have a Class B network with the
network number 172.16.0.0.
• After assessing the needs of your network, you decide
to borrow 8 bits in order to create subnets.
• As you learned earlier, when you borrow 8 bits with a
Class B network, the subnet mask is 255.255.255.0
Creating a Subnet : Boolean AND
operation
• Someone outside the network sends data to the IP
address 172.16.2.120.
• In order to determine where to deliver the data, the
router ANDs this address with the subnet mask.
• When the two numbers are ANDed, the host portion of
the result will always be 0.
• What is left is the network number, including the
subnet.
• Thus, the data is sent to subnet 172.16.2.0, and only
the final router notices that the packet should be
delivered to host 120 in that subnet.
Creating a Subnet : Boolean AND
operation
• Now, imagine that you have the same
network, 172.16.0.0.
• This time, however, you decide to borrow only
7 bits for the subnet field.
• The binary subnet mask for this would be
11111111.11111111.11111110.00000000.
What would this be in dotted decimal
notation?
Creating a Subnet : Boolean AND
operation
• Again, someone outside the network sends data to
host 172.16.2.120.
• In order to determine where to send the data, the
router again ANDs this address with the subnet mask.
• As before, when the two numbers are ANDed, the host
portion of the result is 0.
• So what is different in this second example? Everything
looks the same - at least in decimal.
• The difference is in the number of subnets available,
and the number of hosts that can be in each subnet.
• You can only see this by comparing the two different
subnet masks.
Creating a Subnet : Boolean AND
operation
• With 7 bits in the subnet field, there can be
only 126 subnets.
• How many hosts can there be in each subnet?
• How long is the host field?
• With 9 bits for host numbers, there can be 510
hosts in each of those 126 subnets.
Creating a Subnet : Boolean AND
operation
• The two graphics on this page include something
you'll learn more about later - an alternate way to
express the subnet mask.
• You learned that the 1s of the mask represent
the routing bits - the network plus the subnet.
• 255.255.255.0 indicates there are 24 total routing
bits.
• This is sometimes indicated by following an IP
address with "/24", as in 131.108.3.1 /24 - this
says the same thing as the longer subnet mask.
Creating a Subnet : IP configuration
on a network diagram
• When you configure routers, you must connect each
interface to a different network segment.
• Then each of these segments will become a separate
subnet.
• You must select an address from each different subnet
to assign to the interface of the router that connects to
that subnet.
• Each segment of a network - the actual wires and links
- must have different network/subnet numbers.
• The Figure shows what a network diagram might look
like using a subnetted Class B network.
Creating a Subnet : Host/subnet
schemes
• One of the decisions that you must make
whenever you create subnets is to determine
the optimal number of subnets and hosts
(Note: The number of subnets required in turn
determines the number of hosts available.
• For example, if you borrow 3 bits with a Class
C network, only 5 bits remain for hosts).
Creating a Subnet : Host/subnet
schemes
• You have already learned that you cannot use the
first and last subnet.
• You also cannot use the first and last address
within each subnet - one is the broadcast address
of that subnet, and the other is part of the
network address.
• When you create subnets, you lose quite a few
potential addresses.
• For this reason, network administrators must pay
close attention to the percentage of addresses
that they lose by creating subnets.
Creating a Subnet : Host/subnet
schemes
Example:
If you borrow 2 bits with a Class C network,
you create 4 subnets, each with 64 hosts. Only
2 of the subnets are usable and only 62 hosts
are usable per subnet, leaving 124 usable
hosts out of 254 that were possible before you
chose to use subnets. This means you are
losing 51% of your addresses.
Creating a Subnet : Host/subnet
schemes
• Imagine, this time, that you borrow 3 bits.
• You now have 8 subnets, of which only 6 are
usable, with 30 usable hosts per subnet.
• This gives you a total of 180 usable hosts, down
from 254, but now you are losing only 29% of
your addresses.
• Whenever you create subnets, you need to take
into consideration future network growth and the
percentage of addresses that you would lose by
creating subnets.
Creating a Subnet : Private addresses
• There are certain addresses in each class of IP
address that are not assigned.
• These addresses are called private addresses.
• Private addresses might be used by hosts that
use network address translation (NAT), or a
proxy server, to connect to a public network;
or by hosts that do not connect to the Internet
at all.
Creating a Subnet : Private addresses
• Many applications require connectivity within only one
network and do not need external connectivity.
• In large networks, TCP/IP is often used, even when
network layer connectivity outside the network isn’t
needed. Banks are good examples.
• They may use TCP/IP to connect to automatic teller
machines (ATMs).
• These machines do no connect to the public network,
so private addresses are ideal for them.
• Private addresses can also be used on a network where
there are not enough public addresses available.
Creating a Subnet : Private addresses
• The private addresses can be used together with
a network address translation (NAT) server.
• Either a NAT server or a proxy server to provide
connectivity to all hosts in a network that has
relatively few public addresses available.
• By agreement, any traffic with a destination
address within one of the private address ranges
will NOT be routed on the Internet.