Subnetting and routing

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

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IP addressing• Introduction to IP addressing• Classes of IP addressing• Why Subnet Masks are necessary?• How to create subnet masks

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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.

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What format do IP addresses use?

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How are IP addresses expressed in dotted

notation?

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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.

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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

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How do IP addresses incorporate network

addresses? • Every IP address has two parts. These

are known as the network number and the host number.

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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"

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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

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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

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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

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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.

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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.

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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.

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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.

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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.

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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

&

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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

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– 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

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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

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– 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

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– 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

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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)

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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

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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

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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

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• 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.

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• An example of a Broadcast address

131 181 Host ID Host ID

131 181 255 255

131 181 Host ID Host ID

&

=

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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

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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

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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

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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>

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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

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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

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• 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

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• 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)


Host portion

Subnetwork portion

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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

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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)

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• 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)

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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

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• 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)

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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.

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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.

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The router then looks in its routing table and tries to match the network number

including the subnet with an interface.

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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.

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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

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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

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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.

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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

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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.

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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.

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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

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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

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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}

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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

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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

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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.

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