Lecture 7:
IP Addressing Scheme II
Dr. Mohd Nazri Bin Mohd Warip
High Performance Broadband Networks Research Group
Embedded, Networks and Advanced Computing Research Cluster
School of Computer and Communication Engineering
Universiti Malaysia Perlis
Session 2013/2014
Dr. Mohd Nazri Mohd Warip
Lecture Outline
Subnetting Design
VLSM Design
CIDR
Dr. Mohd Nazri Mohd Warip
November 2013 / EKT 544 Lecture 7
2
Subnet Design Considerations
The key criteria for deployment of an addressing plan:
1. How many total subnets does the organization need today?
2. How many total subnets will the organization need in the future?
3. How many hosts are on the organization’s largest subnet today?
4. How many hosts will there be on the organization’s largest
subnet in the future?
Tx
Dr. Mohd Nazri Mohd Warip
November 2013 / EKT 544 Lecture 7
Rx
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Subnet Example 1
Given network address a.b.c.d/x and required n hosts
number. Find number of q subnet?
Steps to solved.
− Defining the subnet mask / Extended prefix length.
− Defining the subnet number.
− The all-zeros (0s) subnet and all-ones (1s) subnet.
− Defining host addresses for each subnet
− Defining the broadcast address for each subnet.
Tx
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November 2013 / EKT 544 Lecture 7
Rx
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Subnet Example 1
An organization is assigned the network address
193.1.1.0/24 and it needs to defined six subnets. The
largest subnet is required to support 25 hosts.
− Defining the subnet mask / Extended prefix length.
− Defining the subnet number.
− The all-zeros (0s) subnet and all-ones (1s) subnet.
− Defining host addresses for each subnet
− Defining the broadcast address for each subnet.
Tx
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November 2013 / EKT 544 Lecture 7
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Subnet Example 1 (Solution)
Defining the subnet mask / Extended prefix length
− Determine the number bit required: 6 subnets.
− 23=8 and 3 bits are required to enumerate the 8 subnets in the
block and have 2 unused subnets reserved for future growth.
− The organization is subnetting a /24 and extended network prefix
becomes /27 = 255.255.255.224.
Network
Host Number bits
Network Prefix
193.1.1.0/24
11000001
.00000001
.00000001
.00000000
255.255.255.224
11111111
.11111111
.11111111
.11100000
Extended Network Prefix
Tx
Dr. Mohd Nazri Mohd Warip
Subnet Number bits
November 2013 / EKT 544 Lecture 7
Rx
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Subnet Example 1 (Solution)
Defining the subnet Numbers
− The 8 subnets:0 (001), 1 (001), 2 (010), 3 (011), 4 (100), 5 (101),
6 (110), and 7 (111) but required subnet #6 (110)
11000001
00000001
00000001
00000000
193.1.1.0/24
Subnet #0
11000001
.00000001
.00000001
.00000000
193.1.1.0/27
Subnet #1
11000001
.00000001
.00000001
.00100000
193.1.1.32/27
Subnet #2
11000001
.00000001
.00000001
.01000000
193.1.1.64/27
Subnet #3
11000001
.00000001
.00000001
.01100000
193.1.1.96/27
Subnet #4
11000001
.00000001
.00000001
.10000000
193.1.1.128/27
Subnet #5
11000001
.00000001
.00000001
.10100000
193.1.1.160/27
Subnet #6
11000001
.00000001
.00000001
.11000000
193.1.1.192/27
Subnet #7
11000001
.00000001
.00000001
.11100000
193.1.1.224/27
Tx
Extended Network Prefix
Dr. Mohd Nazri Mohd Warip
November 2013 / EKT 544 Lecture 7
Subnet Number bits
Base Net
Rx
Host Number
7
Subnet Example 1 (Solution)
The All-0s Subnet and All-1s Subnet
Subnet Route 193.1.1.0/27
Network
Route
193.1.1.0/24
11000001
.00000001
.00000001
.00000000
27-bit prefix
11000001
.00000001
.00000001
.00000000
.00000001
.00000001
.11111111
27-bit prefix
11000001
.00000001
.00000001
.11111111
All-0s Subnet
− The All-0s subnet (default route) and All-1
Subnet Route 193.1.1.255/27
Network
Route
193.1.1.255/24
Tx
Dr. Mohd Nazri Mohd Warip
11000001
24-bit prefix
November 2013 / EKT 544 Lecture 7
Rx
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All-1s Subnet
24-bit prefix
Subnet Example 1 (Solution)
Defining host address for each subnet
− The All-0s subnet (base network/subnetwork) and All-1s host
number (broadcast address / subnetwork). 25 – 2 = 30 hosts.
11000001
.00000001
.00000001
.01000000
193.1.1.64/27
Host #1
11000001
.00000001
.00000001
.01000001
193.1.1.65/27
Host #2
11000001
.00000001
.00000001
.01000010
193.1.1.66/27
Host #3
11000001
.00000001
.00000001
.01000011
Host #4
11000001
.00000001
.00000001
.01000100
:
:
193.1.1.67/27
193.1.1.68/27
Host #15
:
:
Extended Network Prefix
11000001
.00000001
.00000001
.01001111
:
:
:
Host #29
11000001
.00000001
.00000001
.01011101
193.1.1.93/27
Tx
11000001
.00000001
.00000001
.01011110
193.1.1.94/27
Host #30
Dr. Mohd Nazri Mohd Warip
: Network Prefix
:
Extended
November 2013 / EKT 544 Lecture 7
:
Host Number Bit
Subnet #2
:
193.1.1.79/27
:
Rx
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Subnet Example 1 (Solution)
Defining host address for each subnet
− The valid host address for subnet #6. The 5-bit host number
field: 25 – 2 = 30 hosts.
11000001
.00000001
.00000001
.11000000
193.1.1.192/27
Host #1
11000001
.00000001
.00000001
.11000001
193.1.1.193/27
Host #2
11000001
.00000001
.00000001
.11000010
193.1.1.194/27
Host #3
11000001
.00000001
.00000001
.11000011
Host #4
11000001
.00000001
.00000001
.11000100
:
:
193.1.1.195/27
193.1.1.196/27
Host #15
:
:
Extended Network Prefix
11000001
.00000001
.00000001
.11001111
:
:
:
Host #29
11000001
.00000001
.00000001
.11011101
193.1.1.221/27
Tx
11000001
.00000001
.00000001
.11011110
193.1.1.222/27
Host #30
Dr. Mohd Nazri Mohd Warip
: Network Prefix
:
Extended
November 2013 / EKT 544 Lecture 7
:
Host Number Bit
Subnet #6
:
193.1.1.207/27
:
Rx
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Subnet Example 1 (Solution)
Defining the Broadcast Address for Each Subnet
− The broadcast address for Subnet #n is one less than the base
address for Subnet #(n + 1)
Broadcast Address for Subnet #2 is all-1s host address
11000001
00000001
00000001
01011111
=193.1.1.95
Extended Network Prefix
Broadcast Address for Subnet #6 is all-1s host address
11000001
00000001
00000001
11011111
Tx
Dr. Mohd Nazri Mohd Warip
November 2013 / EKT 544 Lecture 7
=193.1.1.223
Rx
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Subnet Example 2
Given network address a.b.c.d/x. Find n hosts on each
subnet?
Steps to solved.
− Defining the subnet mask / Extended prefix length.
− Defining each of the subnet numbers.
− Defining host addresses for each subnet.
− Defining the broadcast address for each subnet.
Tx
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November 2013 / EKT 544 Lecture 7
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Subnet Example 2
An organization is assigned the network number
140.25.0.0/16 and it must create a set of subnets that
supports up to 60 hosts on each subnet.
Steps to solved.
− Defining the subnet mask / Extended prefix length.
− Defining each of the subnet numbers.
− Defining host addresses for each subnet.
− Defining the broadcast address for each subnet.
Tx
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November 2013 / EKT 544 Lecture 7
Rx
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Subnet Example 2 (Solution)
Defining the subnet mask / Extended prefix length
− Determine the number bit required for 60 hosts on each subnets:
27 – 2 = 126 hosts → 7 bits in the host number field
− The organization is subnetting a /16 and extended network prefix
becomes /25 = 255.255.255.128.
− 9 bits subnet number field. 29 = 512 subnets
Network
Host Number bits
Network Prefix
140.25.0.0/16
10001100
.00011001
.00000000
.00000000
255.255.255.128
11111111
.11111111
.11111111
.10000000
Tx
Dr. Mohd Nazri Mohd Warip
Extended Network Prefix (25-bit)
Subnet Number bits
November 2013 / EKT 544 Lecture 7
Rx
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Subnet Example 2 (Solution)
Defining Each of the Subnet Numbers
− The 512 subnets will be numbered 0 through 511. The 9-bit binary are:
0(0000000002), 1 (0000000012), 3 (0000000112),….., 511 (1111111112)
10001100
00011001
00000000
00000000
140.25.0.0/16
Subnet #0
10001100
00011001
.00000000
.00000000
140.25.0.0/25
Subnet #1
10001100
00011001
.00000000
.10000000
140.25.0.128/25
Subnet #2
10001100
00011001
.00000001
.00000000
140.25.1.0/25
Subnet #3
10001100
00011001
.00000001
.10000000
140.25.1.128/25
Subnet #4
10001100
00011001
.00000010
.00000000
140.25.2.0/25
Subnet #5
10001100
00011001
.00000010
.10000000
140.25.2.128/25
Subnet
#510
10001100
00011001
.11111111
.00000000
140.25.255.0/25
Subnet
#511
10001100
.10000000
Host Number
140.25.255.128/
25
Tx
Tx
00011001
Dr. Mohd Nazri Mohd Warip
Extended
.11111111
November
2013 / EKT 544 Lecture 7
Network
Prefix
Subnet Number bits
Base Net
Rx
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Subnet Example 2 (Solution)
Defining host address for each subnet
− The valid host address for subnet #3. The 7-bit host number
field: 27 – 2 = 126 hosts.
10001100
.00011001
.00000001
.10000000
140.25.1.128/25
Host #1
10001100
.00011001
.00000001
.10000001
140.25.1.129/25
Host #2
10001100
.00011001
.00000001
.10000010
140.25.1.130/25
Host #3
10001100
.00011001
.00000001
.10000011
Host #4
10001100
.00011001
.00000001
.10000100
:
:
140.25.1.131/25
140.25.1.132/25
Host #62
:
:
Extended Network Prefix
10001100
.00011001
.00000001
.10111110
:
:
:
Host #125
10001100
.00011001
.00000001
.11111101
140.25.1.253/25
10001100
.00011001
.00000001
.11111110
140.25.1.254/25
Tx
Host #126
Dr. Mohd Nazri Mohd Warip
: Network Prefix
:
Extended
November 2013 / EKT 544 Lecture 7
:
Host Number Bit
Subnet #3
:
140.25.1.190/25
:
Rx
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Subnet Example 2 (Solution)
Defining the Broadcast Address for Each Subnet
− The broadcast address for Subnet #3 is one less than the base
for Subnet #4 (140.25.2.0).
− The broadcast address for Subnet #n is one less than the base
address for Subnet #(n + 1)
Broadcast Address for Subnet #3 is all-1s host address
10001100
00011001
00000001
11111111
=140.25.1.255
Extended Network Prefix
Tx
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November 2013 / EKT 544 Lecture 7
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Address Assignment and
Summarization
Definition
Description
FLSM (Fixed)
A single subnet mask for the entire IP network number.
VLSM (Variable)
Permits the use of different subnet masks for a network
number.
Summarization
Routing protocols can aggregate subnet routes into one
larger route.
CIDR (Classless)
Implemented in BGP4 to aggregate network routes into a
continuous block of address space that is advertised to the
rest of the internet; decreases the number of BGP routes to
advertise.
Tx
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November 2013 / EKT 544 Lecture 7
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VLSM
Variable Length Subnet Masks (VLSM): the process of
subnetting a subnet to fit your needs.
VLSM helps optimize available address space and
specify a different subnet mask for the same network
number on various subnet.
VLSM further subdivide a network to prevent the wasting
of IP addresses.
Tx
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VLSM
Tx
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VLSM
Classful routing.
− only allows for one subnet mask for all networks
VLSM and classless routing
− This is the process of subnetting a subnet
− More than one subnet mask can be used
− More efficient use of IP addresses as compared to classful IP
addressing
Tx
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VLSM Design Considerations
The key criteria for deployment of VLSM design. The
same set of design decisions must be made at each
level of the hierarchy:
1. How many total subnets does this level need today?
2. How many total subnets will this level need in the future?
3. How many hosts are on this level’s largest subnet today?
4. How many hosts will be on this level’s largest subnet be in the
future?
Tx
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When to Use VLSM?
Tx
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A Waste of IP Address Allocation
Wasting Host Addresses on WAN Connections
Consider class C address 192.168.187.0/27 network
Subnet
Host
3 bits for subnet = 8 - 1
5 bits for hosts
7 useable subnets
30 hosts per subnet
But we don’t want to waste 3 x 30 host subnets on the
WAN connections so we subnet one of the subnets
Must therefore have VLSM enabled routers
Tx
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November 2013 / EKT 544 Lecture 7
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Using VLSM for WAN Connections
Tx
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Subnetting details
Tx
Subnet 6 (/27) further subnetted with mask of /30
to provide 8 subnets in total with 4 hosts per subnet (2 bits).
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VLSM Example
Given network address a.b.c.d/i. Find VLSM
Steps to solved.
− Define the i subnets of a.b.c.d/i.
− Define the host addresses for subnet #p (a.b.e.d/j).
− Define the sub-subnets for subnet #q (a.b.f.d/j).
− Define host addresses for subnet #q - #p (a.b.g.d/k).
− Define the sub-subnet for subnet #q - #q (a.b.h.d/k).
− Define host addresses for subnet #q - #q - #r (a.b.h.r/l)
Tx
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Classless Inter-Domain Routing
CIDR allows routing protocols to summarize multiple
networks, a block of addresses, as a single route.
CIDR deals with Routing Table Explosion Problem
− Networks represented by prefix and mask
− Pre-CIDR: Network with range of 16 contiguous class C blocks
requires 16 entries
− Post-CIDR: Network with range of 16 contiguous class C blocks
requires 1 entry
Solution: Route according to prefix of address, not class
Tx
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November 2013 / EKT 544 Lecture 7
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CIDR Allocation Principles
(RFC1518-1520)
IP address assignment reflects physical topology of network.
Network topology follows continental/national boundaries
− IP addresses should be assigned on this basis
Transit routing domains (TRDs) have unique IP prefix
− carry traffic between routing domains
− interconnected non-hierarchically, cross national boundaries
− Most routing domains single-homed: attached to a single TRD
− Such domains assigned addresses with TRD's IP prefix
− All of the addresses attached to a TRD aggregated into 1table entry
Implementation primarily through BGPv4 (RFC 1520)
Tx
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November 2013 / EKT 544 Lecture 7
Rx
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CIDR and VLSM
Can only subnet a subnet that is empty
Classless InterDomain Routing (CIDR) and
VLSM not only prevents address waste, but also
promotes route aggregation (OR summarization).
Classless routing protocols carry a prefix that
consists of 32-bit IP address and bit mask in the
routing updates
Tx
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November 2013 / EKT 544 Lecture 7
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CIDR and Route Summarization
CIDR = Route Summarization
A supernet summarizes multiple network addresses with
a mask less than the classful mask.
Tx
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November 2013 / EKT 544 Lecture 7
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CIDR and Route Summarization
192.168.0.0/23, 192.168.2.0/23, 192.168.4.0/22, and
192.168.8.0/21 are all subnets of 192.168.0.0/20
Tx
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November 2013 / EKT 544 Lecture 7
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CIDR and Route Summarization
Propagating VLSM and supernet routes requires a
classless routing protocol, because the subnet mask can
no longer be determined by the value of the first octet.
Tx
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November 2013 / EKT 544 Lecture 7
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Longest Prefix Match
CIDR impacts routing & forwarding
Routing tables and routing protocols must carry IP address
and mask
Multiple entries may match a given IP destination address
Example: Routing table may contain
− 205.100.0.0/22 which corresponds to a given supernet
− 205.100.0.0/20 which results from aggregation of a larger number of
destinations into a supernet
− Packet must be routed using the more specific route, that is, the longest
prefix match
Several fast longest-prefix matching algorithms are available
Tx
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November 2013 / EKT 544 Lecture 7
Rx
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IP Routing Protocols
IP routing protocols are characterized by:
− Classful or classless IP addressing/routing
− Static and dynamic routing (distance vector and link state)
− routing metrics
− Variable-Length Subnet Mask (VLSM) to conserve and use
efficiently the total IP addresses allocation
− route summarization across network boundaries
− timers
Tx
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Rx
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IP Routing Protocols
IP routing provides a mechanism to route packets from
different network addresses classified as an inter-domain
or Interior Gateway Protocol (IGP) and intra-domain or
Exterior Gateway Protocol (EGP) routing protocols.
IGP is designed to distribute routing information within
an Autonomous System (AS) and uses the IP address to
establish the route, such as Routing Information Protocol
(RIP), Interior Gateway Routing Protocol (IGRP), and
Open Shortest Path First (OSPF).
EGP is also used to exchange routing information
among different Autonomous Systems (ASs) and
depends on an AS number to construct paths, for
example, Border Gateway Protocol (BGP).
Tx
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References
Kurose, J. F. and Ross, K. W., “Computer Networking A
Top-Down Approach”, 6th. Edition, Pearson, 2012. ISBN:
9780273768968
Alberto Leon-Garcia and Indra Widjaja., “Communication
Networks: Fundamental Concepts and Key
Architectures”, 2nd. Edition, McGraw Hill, 2006.
Tanenbaum, A. S., “Computer Network”, 5th Edition.
Prentice-Hall, 2011.
William Stallings, Data & Computer Communications, 8th.
Edition, Prentice Hall, 2009.
Cisco Systems, http://www.cisco.com
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November 2013 / EKT 544 Lecture 7
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