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SUBNET & ROUTINGSUBNET & ROUTINGSUBNET & ROUTINGSUBNET & ROUTING
Sritrusta SukaridhotoSritrusta Sukaridhoto
LecturerLecturerPens EEPIS-ITSPens EEPIS-ITS
Netmask / Subnet Netmask / Subnet mask tutorialmask tutorial
Netmask / Subnet Netmask / Subnet mask tutorialmask tutorial
IP AddressingIP AddressingIP AddressingIP Addressing
Subnet & Routing 4
IP addressing• Introduction to IP addressing• Classes of IP addressing• Why Subnet Masks are necessary?• How to create subnet masks
Subnet & Routing 5
Why are IP addresses written as bits?
• In order for data to pass along the media, it must first be changed to electrical impulses.
• When a computer receives these electrical impulses, it recognizes two things: the presence of voltage on the wire or the absence of voltage on the wire.
Subnet & Routing 6
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What format do IP addresses use?
Subnet & Routing 8
How are IP addresses expressed in dotted
notation?
Subnet & Routing 9
Why are IP addresses necessary?
• IP addressing makes it possible for data passing over the network media of the Internet to find its destination.
• Because each IP address is a 32-bit value, that means that there are four billion different IP address possibilities.
• IP addresses are hierarchical addresses like phone numbers and zip codes.
Subnet & Routing 10
How do IP addresses make it possible for data sent via the
Internet to find its destination?
• It is because each network connected to the Internet has a unique network number.
• To ensure that each network number on the Internet will always be unique and unlike that of any other number, an organization called the International Network Information Center, or InterNIC
Subnet & Routing 11
How do IP addresses incorporate network
addresses? • Every IP address has two parts. These
are known as the network number and the host number.
Subnet & Routing 12
What are the different classes of networks?
• There are three classes of IP addresses that a company can receive from the InterNIC. The InterNIC reserves class "A" IP addresses for governments throughout the world, class "B" IP addresses for medium size companies, and class "C"
Subnet & Routing 13
IP addressing• Classes of Networks
N H H HClass A:
N N N HClass C:
N N H HClass B:
Network number assigned by NICHost number assigned by Systems Administrator
1 Byte 1 Byte 1 Byte 1 Byte
Subnet & Routing 14
IP addressing• IP Address Bit Patterns
0Network #
Host #Class A:
Bit # 1 2 - 8 9 – 32 (24 bits)
Class A address range1.0.0.0 – 126.0.0.0 (127.0.0.0 is for loopback)Private Class A address: 10.0.0.0Number of hosts: 224 -2 = 16,777,214
8 16 24 32
Subnet & Routing 15
IP addressing• IP Address Bit Patterns
Class B:
Bit #1 3 - 16 17 – 32 (16 bits)
Class B address range128.0.0.0 – 191.255.0.0Private Class B : 172.16.0.0 – 172.31.0.0 Number of hosts: 216 - 2 = 65,534
1 0Network #
Host #
2
8 16 24 32
Subnet & Routing 16
IP addressing• IP Address Bit Patterns
Class C:
Bit #1 4 - 24 25 – 32 (8 bits)
Class C address range192.0.0.0 – 233.255.255.0Private Class C : 192.168.0.0 Number of hosts: 28 - 2 = 254
2
8 16 24 32
1 1 0 Network # Host #
3
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How many classes of Networks are there?
• you have learned about three classes of networks that can be assigned by the InterNIC.
• In fact, there are five classes of networks. However, only three of these are used commercially.
Subnet & Routing 19
What IP addresses are reserved for multicast
purposes and experimental purposes?
• The highest number listed was 223. You may have wondered why the highest value was only 223 and not 255, since there are 255 possible values for an octet.
• in IP addresses the values 224 through 255 are not used in the first octet for networking purposes.
Subnet & Routing 20
What IP addresses are reserved for the Networks?• By convention, in IP addressing schemes,
any IP address that ends in all binary zeroes is reserved for the networkaddress.
• Thus, in a class "A" network, 113.0.0.0 would be the IP address of that network. Routers use a network's IP address when forwarding data on the Internet.
Subnet & Routing 21
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What IP addresses are reserved for broadcasts?
• for the network that is 176.10.0.0, the broadcast address that would be sent out to all devices on that network would be 176.10.255.255.
Subnet & Routing 23
Who assigns subnet addresses?
• As with the host number portion of class "A," class "B," and class "C” addresses, subnet addresses are assigned locally.
• Usually this is done by the network administrator.
Subnet & Routing 24
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IP addressing – Subnet mask
• Subnet Masking - Why?
131 181 115 252IP address
255 255 0 0Netmask
131 181 0 0
10000011
10110101
01110011
11111100
11111111
11111111
00000000
00000000
10000011
10110101
00000000
00000000
=
Network Address
&
Subnet & Routing 26
IP addressing - Subnetting
• Subnetting– Is the act of “borrowing” bits from the
host portion to create smaller networks (called subnetworks)
– Minimum bits that can be borrowed is 2 - why?
– Subnetting is used to reduce the number of broadcast domains
– Communication between these subnetworks is achieved through a router
Subnet & Routing 27
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How are subnet addresses concealed
from outside networks?• Subnets are hidden from outside
networks by using a mask. • These are referred to as subnet masks. • The function of a subnet mask is to tell
devices which part of an address is the network number including the subnet, and which part is the host.
Subnet & Routing 29
What format do subnet masks use?
• Subnet masks use the same format as IP addressing.
• In other words, they are thirty two bits long and divided into four octets.
• Subnet masks have all 1s in the network and subnetwork portion, and all 0s in the host portion.
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How many bits can be borrowed from the host number in class "B" and class "C" networks to create
subnets?
• Because there are only two octets in the host field of a class "B” network, up to fourteen bits can be borrowed to create subnetworks.
• A class "C" network has only one octet in the host field. Therefore, only up to six bits can be borrowed in class "C” networks to create subnetworks.
Subnet & Routing 33
What happens to the subnet mask address if only some of the bits in
an octet are borrowed? • Imagine that you have a class "B” network.
This time however, instead of borrowing all eight bits of the third octet, only seven bits are borrowed to create subnetworks.
• Using binary representation, in this example, the subnet mask would be 11111111.11111111.11111110.00000000.
• Therefore, 255.255.255.0 can no longer be
used as the subnet mask.
Subnet & Routing 34
If only seven bits are borrowed in a class "B" network, what would the subnet mask be in dotted decimal
notation?
• HINT: To convert any eight bit binary number into a decimal number, total the powers of 2
that occur in the number.
Subnet & Routing 35
What determines how many subnetworks can be created by
borrowing bits from the host field?
• Can you figure out all of the possible combinations of 0s and 1s if four bits are borrowed from the host field to create subnetworks?
• 16 from 0000 to 1111. However, you know that 1111 is reserved for broadcast and 0000 means this network.
Subnet & Routing 36
How many subnetworks can be created by borrowing five
bits from the host field? • Answer: Thirty-two
subnetworks or 25 =32 subnetworks can be created by borrowing five bits from the host field.
Subnet & Routing 37
Which numbers in a subnetwork are reserved for
broadcasts? • In previous section, we used an example of
a class "C" network in which three bits are borrowed from the host field. You learned that when three bits are borrowed from the host octet, up to eight subnetworks can be created each having up to thirty-two hosts.
• You also learned that IP addresses ending in all binary 1s are reserved for broadcasts. The same is true for subnetworks.
Subnet & Routing 38
For IP address 197.15.22.160 on the same class "C” network. How would this
be expressed in a binary numbering scheme?
• Answer: If 197.15.22.160 is converted to binary format, it becomes 110001010.00001111.00010110. 10100000.
• The first three bits in the last octet, 101, indicate that this is the sixth subnetwork. As before the remaining bits are all binary 0s. This means that the IP address197.15.22.160 must be one that is reserved for a subnetwork address.
Subnet & Routing 39
Subnet mask• How do we determine how many bits to
“borrow” for a subnet?• Determine the number of sub networks
required• Work from the MOST significant (LHS) bits
of the first octet after the network number and calculate the number of bits needed to create the required number of subnetworks
Subnet & Routing 40
Subnet mask• Example:
– You are given a class B address and you are required to create 1000 subnetworks.
– By working from the LHS of the (first octet after the network number) 3rd octet, calculate the number of bits to equal or slightly exceed 1000. (ie 2x = > 1000)
– This would equate to 210 or 1024-2 networks– Hence you will need to borrow 10 bits from
the host portion to create 1000 subnetworks
Subnet & Routing 41
Subnet mask• Example:
– The subnetmask in this instance would be255.255.255.192
– How many host per network can you obtain from this addressing scheme?
11111111
11111111
11111111
11 000000
10 Bits Borrowed (subnetmask)Natural Class B netmask
6 bits left for hosts
Subnet & Routing 42
Subnet mask• How do we determine how many bits to
“borrow” for a subnet given the number of hosts required?
• Determine the number of hosts required
• Work from the LEAST significant (RHS) bits of the last octet and calculate the number of bits needed to create the required number of subnetworks
Subnet & Routing 43
Subnet mask• Example:
– You are given a class B address and you require 1000 nodes per subnet
– By working from the RHS (last octet) of the 4th octet, calculate the number of bits to equal or slightly exceed 1000. (ie 2x = > 1000)
– This would equate to 210 or 1024-2 networks– Hence you will need to borrow 6 bits from the
host portion to create subnetworks with 1000 hosts each
Subnet & Routing 44
Subnet mask• Example:
– The subnetmask in this instance would be255.255.252.0
– How many subnetworks per network can you obtain from this addressing scheme?
– Note: Do you recognise this address as the student “supernet” address?
11111111
11111111
111111 00
00000000
6 Bits Borrowed (subnetmask)Natural Class B netmask
10 bits required for hosts
Subnet & Routing 45
What about a Supernet?
• A supernet “borrows” bits from the network portion to create contiguous nodes to form a “super network”
• For example– Company A has about 1000 nodes to address. A
class B address would be too big (or may not be available). Solution Supernetting using 4 contiguous class C addresses
203.10.112.0203.10.113.0203.10.114.0203.10.115.0
(All netmasked to 255.255.255.0)
Subnet & Routing 46
What about a Supernet?• By allowing the first 2 bits of the third octet
to be “borrowed”, a virtual class B address can be created.
• A supernet address of 203.10.112.0 – 203.10.115.255 is formed with a subnet mask of 255.255.252.0.
• The Host portion will be expanded from 8 – 10 bits
• Route summarisation can occur to 203.10.112.0/22
Subnet & Routing 47
What about a Supernet?
11001011
00001010
011100 00
00000000
11001011
00001010
011100 01
00000000
11001011
00001010
011100 10
00000000
11001011
00001010
011100 11
11111111
11111111
11111111
111111 00
00000000
Host portionNetwork Portion
203.10.112.0
203.10.115.255
203.10.113.0
203.10.114.0
255.255.252.0
We have expanded the host portion by 2 bits to 10 bits
Subnet & Routing 48
What is a Broadcast Address
• A broadcast address is used to by a node to communicate with ALL nodes in a broadcast domain
• Like the netmask, the broadcast address is “AND” with the network address.
• However, the host portion of the network is identified in a broadcast address
Subnet & Routing 49
What is a Broadcast Address
• A broadcast address does this by inserting all “1’s” in the host portion.
• Eg A natural class B broadcast address would look something like this– N.N.255.255
• If it is not a classful subnetmask, you can determine the broadcast address within each subnet by locating the host portion and setting them to all 1’s.
Subnet & Routing 50
What is a Broadcast Address
• An example of a Broadcast address
131 181 Host ID Host ID
131 181 255 255
131 181 Host ID Host ID
&
=
Subnet & Routing 51
What is a Broadcast Address• An example of a cross boundary subnet
broadcast address with a mask of 255.255.252.0
131 181 11[2,3,4,5] x
131 181 115 255
131 181 SN + H Host ID
&
=
10000011
10110101
011100 00
00000000
10000011
10110101
011100 xx
xxxxxxxx
10000011
10110101
011100 11
11111111
Host portionNetwork Portion SNNetwork Address
Broadcast Address
Host
Subnet & Routing 52
What is a Broadcast Address• An example of a cross boundary subnet
broadcast address with a mask of 255.255.252.0
– In this example, IP addresses•131.181.112.0 – 131.181.115.255
belong to the same subnetwork
10000011
10110101
011100 00
00000000
10000011
10110101
011100 01
00000000
10000011
10110101
011100 10
00000000
10000011
10110101
011100 11
11111111
Host portionNetwork Portion SN131.181.112.0(Network)
131.181.115.255(Broadacast)
131.181.113.0
131.181.114.0
Subnet & Routing 53
Network Address VS
Broadcast address• Remember • A Network address has all the host
bits set to “0”• A Broadcast address has all the host
bits set to “1”• Therefore
– 131.181.112.0 is the network address– 131.181.115.255 is the broadcast address
Subnet & Routing 54
Network Address VS
Broadcast address• This is important when you are doing
ifconfig and routing commands• For example, if a host has an address
131.181.114.10/22• The ifconfig & route commands would be
– ifconfig eth<x> inet 131.181.114.10 netmask 255.255.252.0 broadcast 131.181.115.255
– route add –net 131.181.112.0 netmask 255.255.252.0 dev eth<x>
Subnet & Routing 55
How do you determine Network and Broadcast
address quickly?• There are different subnetting exercises• Given an IP address & mask,
– What is the network/subnetwork address– What is the network/subnetwork broadcast
address– What are the assignable address in that
network/subnetwork– What are all the valid subnet addresses– How many nodes per subnet
Subnet & Routing 56
Algorithm for deriving Subnet information
• Given an IP address, you will usually be given a net/subnetmask
• If you are given the mask– Subtract the mask from 256– This is known as the multiplier– The first number in each multiplier value
is the network number– The broadcast address is the next
multiplier value subtract 1
Subnet & Routing 57
Algorithm for deriving Subnet information
• Eg given the IP address 192.168.0.100 with the subnet mask of 255.255.255.240 Or 192.168.0.100/28– What is the network number– What is the broadcast address– What are the valid IP hosts for the
subnet
Subnet & Routing 58
• Subtract 256 from the netmask– 256 - 240 = 16– This is the multiplier ie the networks are in steps of
16 (16,32,48,64,96,112 etc)• The IP address 192.168.0.100 is in the range of |100 /
16| which is the 6 th subnetwork • The network address is
16* 6 = 96 (01100000b)• The Broadcast address is 96 + 16 - 1
=> 192.168.0.111 (01101111b)– ie (next multiplier – 1)
Algorithm for deriving Subnet information
Host portion
Subnetwork portion
Subnet & Routing 59
Algorithm for deriving Subnet information
11000000
10101000
00000000
0110 0100
11111111
11111111
11111111
1111 0000
11000000
10101000
00000000
0110 0000
11000000
10101000
00000000
0110 1111
Host portionNetwork Portion SNIP address
192.168.0.100
Netmask 255.255.255.240
Network Address192.168.0.96
Broadcast Address192.168.0.111
Subnet & Routing 60
Algorithm for deriving Subnet information• Valid ranges are
– 192.168.0.97 to 192.168.0.110– Number of allowable hosts 97 to 110 (incl) = 14 or [24]16 - 2 = 14
• Remember you cannot use the first address (network address) and the last address (broadcast address) in the range
• The number of allowable networks– [24]16 - 2 = 14 ( ie 4 bits used. If a class B address with the last bit
subnet, then add another 8 bits to give you 212 –2 allowable subnet)
Subnet & Routing 61
Algorithm for deriving Subnet information
• What if the IP range goes over 2 octets• Use the same principal
– Remember octets with all 0’s are considered “boring” and will be assigned the mask of 0
– You will then have to locate the position in the address with both 1’s and 0’s (interesting byte) and use the same algorithm
• Similarly all 1’s are also considered boring and will be given the mask of 255 (eg subnetting the last byte of a class B address)
Subnet & Routing 62
Algorithm for deriving Subnet information • Example
– QUT students’ “supernet” address– 131.181.112.0/22
• Netmask expanded : 255.255.252.0• Last byte is “not interesting” hence we set it to “0” for
network and “1” for broadcast• The third byte is “interesting”
256 – 252 = 4 (multiplier)• Networks are in increments of 4 steps• 112/4 = 28 (the 28th subnetwork). Since there is no
remainder, it is the beginning of the network address
Subnet & Routing 63
Algorithm for deriving Subnet information
• Network address :131.181.112.0
• Netmask :255.255.252.0
• Broadcast :112 + 4 – 1 = 115 =>131.181.115.255
• Number of valid hosts :210 –2 = 1024 – 2 = 1022 hosts
• Number of subnetworks available for this network26 – 2 = 64 –2 =62 subnetworks
Assignable addresses in this subnetwork131.181.112.1 – 131.181.115.254
Remember, the 1st and last addresses cannot be used (Network and broadcast)
Subnet & Routing 64
How does the router handle IP addresses and subnet masks?
• Let's assume that a device on another network with an IP address of 197.15.22.44 wants to send data to another device attached to Cisco's network with an IP address of 131.108.2.2.
• The data is sent out over the Internet until it reaches the router that is attached to Cisco's network.
• The router's job is to determine which one of Cisco's subnetworks the data should be routed to.
Subnet & Routing 65
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when the router performs this ”ANDing" operation, the host
portion falls through.
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The router looks at what is left which is the network number
including the subnetwork.
Subnet & Routing 68
The router then looks in its routing table and tries to match the network number
including the subnet with an interface.
Subnet & Routing 69
How does the "Anding” operation change with different
subnet masks? • Imagine that you have a class "B” network
with the network number 172.16.0.0. • After assessing the needs of his network, the
network administrator has decided to borrow eight bits in order to create subnetworks.
• When eight bits are borrowed to create subnets, the subnet mask is 255.255.255.0.
• Someone outside the network sends data to the IP address 172.16.2.120.
Subnet & Routing 70
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Exercise• Exercise
– You are given an IP address for a host172.168.35.10/20
• What is/are the– 1. Subnet address?– 2. Broadcast address?– 3. The number of useable hosts available for
this subnet?– 4. The number of useable subnets available
for this network?– 5. The assignable address range for this
subnet?Answers
Subnet & Routing 73
Exercise• Exercise
– Your organisation has been assigned a class B IP address of 130.10.0.0
– You require about 2000 subnetworks
• Work out the– 1. Subnet mask required for this subnet– 2. The network and broadcast addresses for
the first 5 useable subnets– 3. The number of hosts for each subnet– 4. The assignable address range of the first
5 useable subnetsAnswers
ROUTINGROUTINGROUTINGROUTING
Subnet & Routing 75
Routing• Routing describes a process where
packets are forwarded from one network to another.
• Routing can be performed by devices such as :– dedicated routers– servers with more than one network interface:
multihomed hosts– switches incorporating a route function.
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Routers• Routing devices typically have more than
one network interface, each called a port.• Routers process datagrams individually,
making routing a processing-intensive operation.
• Dedicated routers offer better performance characteristics compared with multi-homed hosts.
Subnet & Routing 77
Routing Tables• To determining the proper destination
network for datagrams, routers consult an internal table.
• The table consists of records, one per line, each representing a known network.
• Each record includes a set of associated characteristics such as netmask
Subnet & Routing 78
Building Route Tables.• Routing table entries can be built by
two methods:– Static: entries are entered manually by a
network administrator– Dynamic: entries are entered dynamically
by routing protocols. Routers learn destination network addresses by the periodic exchange of route tables between routing devices. Routing protocols use IP to deliver this information.
Subnet & Routing 79
Configuring Routers• Before IP routers can perform the route
function and use routing protocols to exchange route tables, each interface (port) must be correctly numbered with a valid host IP address and netmask.
• The IP address must be selected from within the range for the particular network address. Typically local gateways are located the first address in the valid host range.
Subnet & Routing 80
Routing Protocols• Routing protocols are used by routers to:
– learn the location of destination networks.– determine the best route to reach
networks.• Examples of routing protocols include:
– RIP, Routing Information Protocol– OSPF, Open Shortest Path First– BGP, Border Gateway Protocol
Subnet & Routing 81
Routing Protocols - 2• Routing protocols differ in:
– the way in which they exchange route tables
– determine the route to the destination
– the information that is communicated
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Distance Vector• Distance Vector routing protocols
broadcast the entire route table on a regular basis. RIP2 typically defaults at once every 30 seconds.This creates considerable network traffic.
• They determine the best route path on the basis of the least number number of hops to reach a destination network.
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Link State• Link state protocols only broadcast
changes to route information after an initial entire table has been sent.
• When determining the best path, other factors such as policies (e.g. preferred path) and cost ( time taken, available bandwidth) can influence the choice when multiple paths are available.
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IP Routing• Before a routing device can forward an
IP datagram it must:– examine the Destination Address in the
datagram– use the netmask to identify the network
portion of the packet’s destination address– find a corresponding network address in
the route table and forward the packet to the gateway or interface specified
Subnet & Routing 85
Route Table Fields• To forward IP datagrams, the
router uses the following fields of the the route table:– Destination– Network Mask– Gateway
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Destination • This field lists the networks which are
known to the router. Addresses may have been entered by an administrator, or dynamically learned from the transmissions of other routers.
• Address entries concerned with routing between network addresses will be of the format {<netid>, 0}
Subnet & Routing 87
Netmask• The netmask field provides the router
with the ability to determine the network address of packets being examined.
• A logical AND is performed using the netmask and the destination address. This logically removes the host portion allowing the router to identify the destination
Subnet & Routing 88
Gateway• The Gateway field lists the IP addresses of
the interface where the datagram should be sent (forwarded) to reach the specified Destination.
• This field may contain :– An IP interface address corresponding to an
adjacent router– 0.0.0.0– The address of a interface
Subnet & Routing 89
Destination Hop=0.0.0.0
• A Destination of 0.0.0.0 indicates a directly connected network. Hosts located on this network can be reached using the local network method. If the network is Ethernet, the ARP protocol is used to find the physical address of the node.