Chapter 6
IPv4 Addresses – Part 3
CIS 81 Networking Fundamentals
Rick Graziani
Cabrillo College
graziani@cabrillo.edu
Last Updated: 4/13/2008
Topics





Calculating the number subnets/hosts needed
VLSM (Variable Length Subnet Masks)
Classful Subnetting
IPv6
ICMP: Ping and Traceroute
2
Calculating the number subnets/hosts
needed
Calculating the number subnets/hosts needed
172.16.1.0
255.255.255.0
Network
Host
 Network 172.16.1.0/24
 Need:
 As many subnets as possible, 60 hosts per subnet
4
Calculating the number subnets/hosts needed
Number of hosts per subnet
172.16.1. 0 0 0 0 0 0 0 0
255.255.255. 0 0 0 0 0 0 0 0
6 host bits
Network
Host
 Network 172.16.1.0/24
 Need:
 As many subnets as possible, 60 hosts per subnet
5
Calculating the number subnets/hosts needed
Number of subnets
172.16.1. 0 0 0 0 0 0 0 0
255.255.255. 1 1 0 0 0 0 0 0
255.255.255.192
6 host bits
Network
Host
 Network 172.16.1.0/24
 Need:
 As many subnets as possible, 60 hosts per subnet
 New Subnet Mask: 255.255.255.192 (/26)
 Number of Hosts per subnet: 6 bits, 64-2 hosts, 62 hosts
 Number of Subnets: 2 bits or 4 subnets
6
Calculating the number subnets/hosts needed
172.16.1.0
255.255.255.0
Network
Host
 Network 172.16.1.0/24
 Need:
 As many subnets as possible, 12 hosts per subnet
7
Calculating the number subnets/hosts needed
Number of hosts per subnet
172.16.1. 0 0 0 0 0 0 0 0
255.255.255. 0 0 0 0 0 0 0 0
4 host bits
Network
Host
 Network 172.16.1.0/24
 Need:
 As many subnets as possible, 12 hosts per subnet
8
Calculating the number subnets/hosts needed
Number of hosts per subnet
Number of subnets
172.16.1. 0 0 0 0 0 0 0 0
255.255.255. 1 1 1 1 0 0 0 0
255.255.255.240
4 host bits
Network
Host
 Network 172.16.1.0/24
 Need:
 As many subnets as possible, 12 hosts per subnet
 New Subnet Mask: 255.255.255.240 (/28)
 Number of Hosts per subnet: 4 bits, 16-2 hosts, 14 hosts
 Number of Subnets: 4 bits or 16 subnets
9
Calculating the number subnets/hosts needed
172.16.1.0
255.255.255.0
Network
Host
 Network 172.16.1.0/24
 Need:
 Need 6 subnets, as many hosts per subnet as possible
10
Calculating the number subnets/hosts needed
Number of subnets
172.16.1. 0 0 0 0 0 0 0 0
255.255.255. 0 0 0 0 0 0 0 0
3 subnet bits
Network
Host
 Network 172.16.1.0/24
 Need:
 Need 6 subnets, as many hosts per subnet as possible
11
Calculating the number subnets/hosts needed
Number of hosts per subnet
Number of subnets
172.16.1. 0 0 0 0 0 0 0 0
255.255.255. 1 1 1 0 0 0 0 0
3 subnet bits
Network
255.255.255.224
Host
 Network 172.16.1.0/24
 Need:
 Need 6 subnets, as many hosts per subnet as possible
 New Subnet Mask: 255.255.255.224 (/27)
 Number of Hosts per subnet: 5 bits, 32-2 hosts, 30 hosts
 Number of Subnets: 3 bits or 8 subnets
12
VLSM (Variable Length Subnet Masks)
VLSM
 If you know how to subnet, you can do VLSM.
 Example: 10.0.0.0/8
 Subnet in /16 subnets:
 10.0.0.0/16
 10.1.0.0/16
 10.2.0.0/16
 10.3.0.0/16
 Etc.
 Subnet one of the subnets (10.1.0.0/16)
 10.1.0.0/24
 10.1.1.0/24
 10.1.2.0/24
 10.1.3.0/24
 etc
14
VLSM
Host can only be a member
of the subnet. Host can NOT
be a member of the network
that was subnetted.
YES!
10.2.1.55/24
10.2.1.55/16
All other /16
subnets are still
available for use
as /16 networks or
to be subnetted.
NO!
15
VLSM – Using the chart
 This chart can be used to help
determine subnet addresses.
 This can any octet.
 We’ll keep it simple and make it the
fourth octet.
 Network: 172.16.1.0/24
 What if we needed 10 subnets with a
minimum of 12 hosts?
 What would the Mask be?
 What would the addresses of each
subnet be?
 What would the range of hosts be for
each subnet?
16
VLSM – Using the chart
 Network: 172.16.1.0/24
 What if we needed 5 subnets?
 What would the Mask be?
 255.255.255.240 (/28)
 What would the addresses of each subnet be?
 172.16.1.0/28
 172.16.1.32/28
 172.16.1.64/28
 172.16.1.96/28
 172.16.1.128/28
 172.16.1.160/28
 172.16.1.192/28
 172.16.1.224/28
 What would the range of valid hosts for each subnet?
 172.16.1.0/26: 172.16.1.1-172.16.1.31
 172.16.1.32/26: 172.16.1.33-172.16.1.62
 172.16.1.64/26: 172.16.1.65-172.16.1.94
 172.16.1.96/26: 172.16.1.97-172.16.1.126
 Etc.
17
16 /30 subnets
VLSM – Using the chart
 What if we needed several (four) /30 subnets for our
serial links?
 Take one of the /27 subnets and subnet it again into
/30 subnets.
Still
have 7
/27
subnets
16 /30 subnets
18
Classful Subnetting
Classful IP Addressing
 In the early days of the Internet, IP addresses were allocated to
organizations based on request rather than actual need.
 When an organization received an IP network address, that address was
associated with a “Class”, A, B, or C.
 This is known as Classful IP Addressing
 The first octet of the address determined what class the network belonged
to and which bits were the network bits and which bits were the host bits.
 There were no subnet masks.
 It was not until 1992 when the IETF introduced CIDR (Classless
Interdomain Routing), making the address class meaning less.
 This is known as Classless IP Addressing.
 For now, all you need to know is that today’s networks are classless, except
for some things like the structure of Cisco’s IP routing table and for those
networks that still use Classful routing protocols.
 You will learn more about this is CIS 82, CIS 83 and CIS 185.
20
IPv4 Address Classes
21
Address Classes
1st octet
2nd octet
3rd octet
4th octet
Class A
Network
Host
Host
Host
Class B
Network Network
Host
Host
Class C
Network Network Network
Host
N = Network number assigned by ARIN
(American Registry for Internet Numbers)
H = Host number assigned by administrator
22
Class A addresses
Default Mask: 255.0.0.0 (/8)
First octet is between 0 – 127, begins with 0
Network
Number
between 0 - 127
Host
Host
Host
8 bits
8 bits
8 bits
With 24 bits available for hosts,
there a 224 possible addresses.
That’s 16,777,216 nodes!
 There are 126 class A addresses.
 0 and 127 have special meaning and are not used.
 16,777,214 host addresses, one for network address and one for broadcast address.
 Only large organizations such as the military, government agencies, universities, and
large corporations have class A addresses.
 For example ISPs have 24.0.0.0 and 63.0.0.0
 Class A addresses account for 2,147,483,648 of the possible IPv4 addresses.
 That’s 50 % of the total unicast address space, if classful was still used in the Internet!
23
Class B addresses
Default Mask: 255.255.0.0 (/16)
First octet is between 128 – 191, begins with 10
Network Network
Number
between
128 - 191
Host
Host
8 bits
8 bits
With 16 bits available for hosts,
there a 216 possible addresses.
That’s 65,536 nodes!
 There are 16,384 (214) class B networks.
 65,534 host addresses, one for network address and one for broadcast
address.
 Class B addresses represent 25% of the total IPv4 unicast address space.
 Class B addresses are assigned to large organizations including corporations
(such as Cisco, government agencies, and school districts).
24
Class C addresses
Default Mask: 255.255.255.0 (/24)
First octet is between 192 – 223, begins with 110
Network Network Network
Host
8 bits
Number
between
192 - 223
With 8 bits available for hosts,
there a 28 possible addresses.
That’s 256 nodes!
 There are 2,097,152 possible class C networks.
 254 host addresses, one for network address and one for broadcast
address.
 Class C addresses represent 12.5% of the total IPv4 unicast address
space.
25
IPv4 Address Classes
 No medium size host networks
 In the early days of the Internet, IP addresses were allocated to
organizations based on request rather than actual need.
26
Network based on first octet
 The network portion of the IP address was dependent upon the first octet.
 There was no “Base Network Mask” provided by the ISP.
 The network mask was inherent in the address itself.
27
IPv4 Address Classes
Class D Addresses
 A Class D address begins with binary 1110 in the first octet.
 First octet range 224 to 239.
 Class D address can be used to represent a group of hosts called a host
group, or multicast group.
Class E Addresses
First octet of an IP address begins with 1111
 Class E addresses are reserved for experimental purposes and should not
be used for addressing hosts or multicast groups.
28
Fill in the information…
1. 192.168.1.3
Class _____
Default Mask:______________
Network: _________________
Broadcast: ________________
Hosts: _________________ through ___________________
2. 1.12.100.31
Class ______ Default Mask:______________
Network: _________________
Broadcast: ________________
Hosts: _________________ through _____________________
3. 172.30.77.5
Class ______ Default Mask:______________
Network: _________________
Broadcast: ________________
Hosts: _________________ through _____________________
29
Fill in the information…
1. 192.168.1.3
Class C
Default Mask: 255.255.255.0
Network: 192.168.1.0
Broadcast: 192.168.1.255
Hosts: 192.168.1.1 through 192.168.1.254
2. 1.12.100.31
Network: 1.0.0.0
Hosts: 1.0.0.1
Class A
through
Default Mask: 255.0.0.0
Broadcast: 1.255.255.255
1.255.255.254
3. 172.30.77.5
Class B
Default Mask: 255.255.0.0
Network: 172.30.0.0
Broadcast: 172.30.255.255
Hosts: 172.30.0.1. through 172.30.255.254
30
Class separates network from host bits
 The Class determines the Base Network Mask!
1. 192.168.1.3
Class C
Default Mask: 255.255.255.0
Network: 192.168.1.0
2. 1.12.100.31
Class A
Default Mask: 255.0.0.0
Network: 1.0.0.0
3. 172.30.77.5
Class B
Default Mask: 255.255.0.0
Network: 172.30.0.0
31
Know the classes!
Class
First
Bits
First
Octet
Network
Bits
Host
Bits
A
0
0 – 127
8
24
B
10
128 - 191
16
16
C
110
192 - 223
24
8
D
1110
224 – 239
E
1111
240 - 255
32
IP addressing crisis
 Address Depletion
 Internet Routing Table Explosion
33
IPv4 Addressing
Subnet Mask
 One solution to the IP address shortage was thought to be the subnet
mask.
 Formalized in 1985 (RFC 950), the subnet mask breaks a single class A, B
or C network in to smaller pieces.
 This does allow a network administrator to divide their network into subnets.
 Routers still associated an network address with the first octet of the IP
address.
34
All Zeros and All Ones Subnets
Using the All Ones Subnet
 There is no command to enable or disable the use of the all-ones subnet,
it is enabled by default.
Router(config)#ip subnet-zero
 The use of the all-ones subnet has always been explicitly allowed and the
use of subnet zero is explicitly allowed since Cisco IOS version 12.0.
RFC 1878 states, "This practice (of excluding all-zeros and all-ones
subnets) is obsolete! Modern software will be able to utilize all definable
networks." Today, the use of subnet zero and the all-ones subnet is
generally accepted and most vendors support their use, though, on
certain networks, particularly the ones using legacy software, the use of
subnet zero and the all-ones subnet can lead to problems.
CCO: Subnet Zero and the All-Ones Subnet
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note091
86a0080093f18.shtml
35
Long Term Solution: IPv6 (coming)
 IPv6, or IPng (IP – the Next Generation) uses a 128-bit address
space, yielding
340,282,366,920,938,463,463,374,607,431,768,211,456
possible addresses.
 IPv6 has been slow to arrive
 IPv6 requires new software; IT staffs must be retrained
 IPv6 will most likely coexist with IPv4 for years to come.
 Some experts believe IPv4 will remain for more than 10 years.
36
Short Term Solutions: IPv4 Enhancements
Discussed in CIS 83 and CIS 185
 CIDR (Classless Inter-Domain Routing) – RFCs 1517, 1518, 1519, 1520
 VLSM (Variable Length Subnet Mask) – RFC 1009
 Private Addressing - RFC 1918
 NAT/PAT (Network Address Translation / Port Address Translation) – RFC
 More later when we discuss TCP
37
11111111.00000000.00000000.00000000 /8 (255.0.0.0)
16,777,216 host addresses
11111111.10000000.00000000.00000000 /9 (255.128.0.0)
8,388,608 host addresses
ISPs
no longer restricted to
11111111.11000000.00000000.00000000
/10 (255.192.0.0)
4,194,304 host addresses
three
classes. Can now
11111111.11100000.00000000.00000000
/11 (255.224.0.0)
2,097,152 host addresses
allocate
a large range of
11111111.11110000.00000000.00000000
/12 (255.240.0.0)
1,048,576 host addresses
network
addresses based
11111111.11111000.00000000.00000000
/13 (255.248.0.0)
524,288 host addresses
on11111111.11111100.00000000.00000000
customer requirements
/14 (255.252.0.0)
262,144 host addresses
11111111.11111110.00000000.00000000 /15 (255.254.0.0) 131,072 host addresses
11111111.11111111.00000000.00000000 /16 (255.255.0.0) 65,536 host addresses
11111111.11111111.10000000.00000000 /17 (255.255.128.0)
32,768 host addresses
11111111.11111111.11000000.00000000 /18 (255.255.192.0)
16,384 host addresses
11111111.11111111.11100000.00000000 /19 (255.255.224.0)
8,192 host addresses
11111111.11111111.11110000.00000000 /20 (255.255.240.0)
4,096 host addresses
11111111.11111111.11111000.00000000 /21 (255.255.248.0)
2,048 host addresses
11111111.11111111.11111100.00000000 /22 (255.255.252.0)
1,024 host addresses
11111111.11111111.11111110.00000000 /23 (255.255.254.0)
512 host addresses
11111111.11111111.11111111.00000000 /24 (255.255.255.0)
256 host addresses
11111111.11111111.11111111.10000000 /25 (255.255.255.128)
128 host addresses
11111111.11111111.11111111.11000000 /26 (255.255.255.192)
64 host addresses
11111111.11111111.11111111.11100000 /27 (255.255.255.224)
32 host addresses
11111111.11111111.11111111.11110000 /28 (255.255.255.240)
16 host addresses
11111111.11111111.11111111.11111000 /29 (255.255.255.248)
8 host addresses
11111111.11111111.11111111.11111100 /30 (255.255.255.252)
4 host addresses
11111111.11111111.11111111.11111110 /31 (255.255.255.254)
2 host addresses
38
11111111.11111111.11111111.11111111 /32 (255.255.255.255)
“Host Route”
Active BGP entries – March, 2006
http://bgp.potaroo.net/
39
ISP/NAP Hierarchy - “The Internet: Still hierarchical after all
these years.” Jeff Doyle (Tries to be anyways!)
NAP (Network Access Point)
Network
Service
Provider
Regional
Service
Provider
ISP
Subscribers
ISP
Subscribers
ISP
Subscribers
Network
Service
Provider
Regional
Service
Provider
Regional
Service
Provider
ISP
ISP
Subscribers
Subscribers
Regional
Service
Provider
ISP
Subscribers
ISP
Subscribers
ISP
Subscribers
40
IPv6
Why Do We Need a Larger Address Space?
 Internet population
 Approximately 973 million users in November 2005
 Emerging population and geopolitical and address space
 Mobile users
 PDA, pen-tablet, notepad, and so on
 Approximately 20 million in 2004
 Mobile phones
 Already 1 billion mobile phones delivered by the industry
 Transportation
 1 billion automobiles forecast for 2008
 Internet access in planes – Example: Lufthansa
 Consumer devices
 Sony mandated that all its products be IPv6-enabled by 2005
 Billions of home and industrial appliances
42
IP Address Allocation History
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
1980
1985
1990
1995
2000
2005
2010
1981, IPv4 Protocol was published.
1985, 1/16 of IPv4 address space in use.
2001, 2/3 of IPv4 address space in use.
43
Larger Address Space
IPv4
 32 bits or 4 bytes long
4,200,000,000 possible addressable nodes
IPv6
 128 bits or 16 bytes: four times the bits of IPv4
3.4 * 1038 possible addressable nodes
340,282,366,920,938,463,374,607,432,768,211,456
5 * 1028 addresses per person
50,000,000,000,000,000,000,000,000,000
44
Larger Address Space Enables Address
Aggregation
 Aggregation of prefixes announced in the global routing table
 Efficient and scalable routing
45
IPv6
 Address assignment features: Using DHCP and Stateless
Autoconfiguration.
 Built-in Support for Mobility: IPv6 supports mobility such that IPv6
hosts can move around the Internetwork, retain their IPv6 address and
without losing current application sessions.
 Aggregation: IPv6’s huge address space makes for much easier
aggregation of blocks of addresses in the Internet, making routing in
the Internet more efficient.
 No need for NAT/PAT: The huge public IPv6 address space removes
the need for NAT/PAT, which avoids some NAT-induced application
problems and makes for more efficient routing.
 No Broadcasts: IPv6 does not use layer 3 broadcast addresses,
instead relying on multicasts to reach multiple hosts.
 Transition tools: IPv6 has many rich tools to help with the transition
from IPv4 to IPv6.
46
Three types of IPv6 Addresses
The three types of IPv6 address follow:
1. Unicast
 Global Unicast
 Link Local Unicast
 Unique Local Unicast
2. Multicast
3. Anycast


Unlike IPv4, there is no IPv6 broadcast address.
There is, however, an "all nodes" multicast address, which
serves essentially the same purpose as a broadcast address.
47
Unicast Addresses
 A unicast address is an address that identifies a single device.
 A global unicast address is a unicast address that is globally unique.
 Has global scope.
 Globally unique and can therefore be routed globally with no modification.
48
Global Unicast Addresses
Replaced
with
 Note: This format, specified in RFC 3587, obsoletes and simplifies
an earlier format that divided the IPv6 unicast address into Top
Level Aggregator (TLA), Next-Level Aggregator (NLA), and other
fields. However, you should be aware that this obsolescence is
relatively recent and you are likely to encounter some books and
documents that show the old IPv6 address format.
49
Unicast Addresses




The host portion of the address is called the Interface ID.
Host can have more than one IPv6 interface
Address more correctly identifies an interface on a host than a host itself.
A single interface can have multiple IPv6 addresses, and can have an IPv4
address in addition.
50
Unicast Addresses
 Another big difference between IPv4 addresses and IPv6 addresses:
location of the Subnet Identifier
 Subnet Identifier is part of the network portion of the address rather than
the host portion.
51
Unicast Addresses
 The Interface ID is a consistent size for all IPv6 addresses, simplifying the
parsing of the address.
 And making the Subnet ID a part of the network portion creates a clear
separation of functions:
 The network portion provides the location of a device down to the specific
data link
and
 the host portion provides the identity of the device on the data link.
52
Background
 IPv4 will exist for some time, as the transition begins to IPv6.
 Other new protocols have been developed in support of IPv6:
 Routing protocols (OSPFv3) so routers can learn about IPv6
network addresses.
 ICMPv6
53
ICMP
55
ICMP: Ping and Trace
Ethernet Header
(Layer 2)
Ethernet
Destination
Address
(MAC)
Ethernet
Source
Address
(MAC)
Frame
Type
IP Header
(Layer 3)
ICMP Message
(Layer 3)
Source IP Add.
Dest. IP Add.
Protocol field
Type
0 or 8
Code
0
Ether.
Tr.
Checksum
ID
Seq.
Num.
Data
FCS
Partial list
ICMP (Internet Control Message Protocol)
 ICMP: A Layer 3 protocol
 Used for sending messages
 Encapsulated in a Layer 3, IP packet
 Uses Type and Code fields for various messages
57
ICMP
Ethernet Header
(Layer 2)
Ethernet
Destination
Address
(MAC)
Ethernet
Source
Address
(MAC)
Frame
Type
IP Header
(Layer 3)
ICMP Message
(Layer 3)
Source IP Add.
Dest. IP Add.
Protocol field
Type
0 or 8
Code
0
Ether.
Tr.
Checksum
ID
Seq.
Num.
Data
FCS
Unreachable Destination or Service
 Used to notify a host that the destination or service is unreachable.
 When a host or router receives a packet that it cannot deliver, it may send
an ICMP Destination Unreachable packet to the host originating the
packet.
 The Destination Unreachable packet will contain codes that indicate why
the packet could not be delivered.
From a router:
 0 = network unreachable – Does not have a route in the routing table
 1 = host unreachable – Has a route but can’t find host. (end router)
From a host:
 2 = protocol unreachable
 3 = port unreachable
 Service is not available because no daemon is running providing
the service or because security on the host is not allowing access
to the service.
58
172.30.1.20
172.30.1.25
59
Ethernet Header
(Layer 2)
Ethernet
Destination
Address
(MAC)
Ethernet
Source
Address
(MAC)
Frame
Type
IP Header
(Layer 3)
ICMP Message
(Layer 3)
Source IP Add.
Dest. IP Add.
Protocol field
Type
0 or 8
Code
0
Ether.
Tr.
Checksum
ID
Seq.
Num.
Data
FCS
Ping
 Uses ICMP message encapsulated within an IP Packet
 Protocol field = 1
 Does not use TCP or UDP
Format
 ping ip address (or ping <cr> for extended ping)
 ping 172.30.1.25
60
Ethernet Header
(Layer 2)
Ethernet
Destination
Address
(MAC)
Ethernet
Source
Address
(MAC)
Frame
Type
IP Header
(Layer 3)
ICMP Message - Echo Request
(Layer 3)
Source IP
Add.
172.30.1.20
Dest. IP Add.
172.30.1.25
Protocol field
1
Type
8
Code
0
Checksum
ID
Seq.
Num.
Ether.
Tr.
Data
FCS
Echo Request
 The sender of the ping, transmits an ICMP message, “Echo Request”
Echo Request - Within ICMP Message
 Type = 8
 Code = 0
61
Ethernet Header
(Layer 2)
Ethernet
Destination
Address
(MAC)
Ethernet
Source
Address
(MAC)
Frame
Type
IP Header
(Layer 3)
ICMP Message - Echo Reply
(Layer 3)
Source IP
Add.
172.30.1.25
Dest. IP Add.
172.30.1.20
Protocol field
1
Type
0
Code
0
Checksum
ID
Ether.
Tr.
Seq.
Num.
Data
FCS
Echo Reply
 The IP address (destination) of the ping, receives the ICMP message,
“Echo Request”
 The ip address (destination) of the ping, returns the ICMP message, “Echo
Reply”
Echo Reply - Within ICMP Message
 Type = 0
 Code = 0
62
Ping example
63
Pings
may fail
Q: Are pings forwarded by routers?
A: Yes! This is why you can ping devices all over the Internet.
Q: Do all devices forward or respond to pings?
A: No, this is up to the network administrator of the device. Devices,
including routers, can be configured not to reply to pings (ICMP echo
requests). This is why you may not always be able to ping a device. Also,
routers can be configured not to forward pings destined for other devices.
64
Traceroute
 Traceroute is a utility that records the route (router IP addresses) between
two devices on different networks.
65
Tracroute
 http://en.wikipedia.org/wiki/Traceroute
 On modern Unix and Linux-based operating systems, the traceroute utility
by default uses UDP datagrams with a destination port number starting at
33434.
 The traceroute utility usually has an option to specify use of ICMP echo
request (type 8) instead.
 The Windows utility uses ICMP echo request, better known as ping
packets.
 Some firewalls on the path being investigated may block UDP probes but
allow the ICMP echo request traffic to pass through.
 There are also traceroute implementations sending out TCP packets, such
as tcptraceroute or Layer Four Trace.
 In Microsoft Windows, traceroute is named tracert.
 A new utility, pathping, was introduced with Windows NT, combining ping
and traceroute functionality. All these traceroutes rely on ICMP (type 11)
packets coming back.
66
Trace (Traceroute)
 Trace ( Cisco = traceroute, tracert,…) is used to trace the probable path a
packet takes between source and destination.
 Probable, because IP is a connectionless protocol, and different packets may
take different paths between the same source and destination networks,
although this is not usually the case.
 Trace will show the path the packet takes to the destination, but the return path
may be different.
 This is more likely the case in the Internet, and less likely within your own
autonomous system.
 Linux/Unix Systems
 Uses ICMP message within an IP Packet
 Both are layer 3 protocols.
 Uses UDP as a the transport layer.
 We will see why this is important in a moment.
67
Trace
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
.2
RTD
.1
.2
Format (trace, traceroute, tracert)
 RTA# traceroute ip address
RTA# traceroute 192.168.10.2
68
Trace
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
.2
RTD
.1
.2
DA = 192.168.10.2, TTL = 1
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
……
IP Header
(Layer 3)
Source IP
Add.
10.0.0.1
Dest. IP Add.
192.168.10.2
Protocol field
1
TTL
1
ICMP Message - Echo Request (trace)
Type
8
Chk
sum
ID
Seq.
Num
Data
UDP
(Layer 4)
DestPort
35,000
DataLink
Tr.
FCS
Code
0
How it works (using UDP) - Fooling the routers & host!
 Traceroute uses ping (echo requests)
 Traceroute sets the TTL (Time To Live) field in the IP Header, initially to “1”
 When a router receives an IP Packet, it decrements the TTL by 1.
 If the TTL is 0, it will not forward the IP Packet, and send back to the source
an ICMP “time exceeded” message.
69
Trace
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
.2
RTD
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
….
IP Header
(Layer 3)
Source IP
Add.
10.0.0.2
Dest. IP Add.
10.0.0.1
Protocol field
1
ICMP Message - Time Exceeded
Type
11
Code
0
Chk
sum
ID
Seq
.
Nu
m.
Data
DataLink
Tr.
FCS
RTB - TTL:
 When a router receives an IP Packet, it decrements the TTL by 1.
 If the TTL is 0, it will not forward the IP Packet, and send back to the source
an ICMP “time exceeded” message.
 ICMP Message: Type = 11, Code = 0
70
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
.2
RTD
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
….
IP Header
(Layer 3)
Source IP
Add.
10.0.0.2
Dest. IP Add.
10.0.0.1
Protocol field
1
ICMP Message - Time Exceeded
Type
11
Code
0
Chk
sum
ID
Seq
.
Nu
m.
Data
DataLink
Tr.
FCS
RTB
 Sends back a ICMP Time Exceeded message back to the source, using its
IP address for the source IP address.
 Router B’s IP header includes its own IP address (source IP) and the sending
host’s IP address (dest. IP).
71
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
.2
RTD
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
….
IP Header
(Layer 3)
Source IP
Add.
10.0.0.2
Dest. IP Add.
10.0.0.1
Protocol field
1
ICMP Message - Time Exceeded
Type
11
Chk
sum
Code
0
ID
Seq
.
Nu
m.
Data
DataLink
Tr.
FCS
RTA, Sending Host
 The traceroute program of the sending host (RTA) will use the source IP
address of this ICMP Time Exceeded packet to display at the first hop.
RTA# traceroute 192.168.10.2
Type escape sequence to abort.
Tracing the route to 192.168.10.2
1 10.0.0.2 4 msec 4 msec 4 msec
72
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
.2
RTD
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
DA = 192.168.10.2, TTL = 2
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
……
IP Header
(Layer 3)
Source IP
Add.
10.0.0.1
Dest. IP Add.
192.168.10.2
Protocol field
1
TTL
2
ICMP Message - Echo Request (trace)
Type
8
Chk
sum
ID
Seq.
Num
Data
UDP
(Layer 4)
DestPort
35,000
DataLink
Tr.
FCS
Code
0
RTA
 The traceroute program increments the TTL by 1 (now 2 ) and resends the
ICMP Echo Request packet.
73
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
.2
RTD
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
DA = 192.168.10.2, TTL = 2
ICMP Time Exceeded, SA = 172.16.0.2
RTB
 This time RTB decrements the TTL by 1 and it is NOT 0. (It is 1.)
 So it looks up the destination ip address in its routing table and forwards it on to
the next router.
RTC
 RTC however decrements the TTL by 1 and it is 0.
 RTC notices the TTL is 0 and sends back the ICMP Time Exceeded message
back to the source.
 RTC’s IP header includes its own IP address (source IP) and the sending host’s
IP address (destination IP address of RTA).
 The sending host, RTA, will use the source IP address of this ICMP Time
Exceeded message to display at the second hop.
74
10.0.0.0/8
172.16.0.0/16
RTA
192.168.10.0/24
RTB
.1
.2
RTC
.1
RTD
.2
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
DA = 192.168.10.2, TTL = 2
ICMP Time Exceeded, SA = 172.16.0.2
RTA to RTB
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
……
IP Header
(Layer 3)
Source IP
Add.
10.0.0.1
Dest. IP Add.
192.168.10.2
Protocol field
1
TTL
2
ICMP Message - Echo Request (trace)
Type
8
Chk
sum
ID
Seq.
Num
Data
UDP
(Layer 4)
DestPort
35,000
DataLink
Tr.
FCS
Code
0
RTB to RTC
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
……
.
IP Header
(Layer 3)
Source IP
Add.
10.0.0.1
Dest. IP Add.
192.168.10.2
Protocol field
1
TTL
1
ICMP Message - Echo Request (trace)
Type
8
Chk
sum
ID
Seq.
Num
Data
UDP
(Layer 4)
DestPort
35,000
DataLink
Tr.
FCS
Code
0
RTC to RTA
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
….
IP Header
(Layer 3)
Source IP
Add.
172.16.0.2
Dest. IP Add.
10.0.0.1
Protocol field
1
ICMP Message - Time Exceeded
Type
11
Code
0
Chk
sum
ID
Seq
.
Nu
m.
Data
DataLink
Tr.
FCS
75
10.0.0.0/8
172.16.0.0/16
RTA
192.168.10.0/24
RTB
.1
.2
RTC
.1
.2
RTD
.1
.2
ICMP Message - Time Exceeded
DataLink
Tr.
FCS
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
DA = 192.168.10.2, TTL = 2
ICMP Time Exceeded, SA = 172.16.0.2
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
….
IP Header
(Layer 3)
Source IP
Add.
172.16.0.2
Dest. IP Add.
10.0.0.1
Protocol field
1
Type
11
Code
0
Chk
sum
ID
Seq
.
Nu
m.
Data
The sending host, RTA:
 The traceroute program uses this information (Source IP Address) and
displays the second hop.
RTA# traceroute 192.168.10.2
Type escape sequence to abort.
Tracing the route to 192.168.10.2
1 10.0.0.2 4 msec 4 msec 4 msec
2 172.16.0.2 20 msec 16 msec 16 msec
76
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
RTD
.2
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
DA = 192.168.10.2, TTL = 2
ICMP Time Exceeded, SA = 172.16.0.2
DA = 192.168.10.2, TTL = 3
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
……
IP Header
(Layer 3)
Source IP
Add.
10.0.0.1
Dest. IP Add.
192.168.10.2
Protocol field
1
TTL
3
ICMP Message - Echo Request (trace)
Type
8
Chk
sum
ID
Seq.
Num
Data
UDP
(Layer 4)
DestPort
35,000
DataLink
Tr.
FCS
Code
0
The sending host, RTA:
 The traceroute program increments the TTL by 1 (now 3 ) and resends the
Packet.
77
10.0.0.0/8
172.16.0.0/16
RTA
192.168.10.0/24
RTB
.1
RTC
.2
.1
RTD
.2
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
DA = 192.168.10.2, TTL = 2
ICMP Time Exceeded, SA = 172.16.0.2
DA = 192.168.10.2, TTL = 3
RTA to RTB
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
……
IP Header
(Layer 3)
Source IP
Add.
10.0.0.1
Dest. IP Add.
192.168.10.2
Protocol field
1
TTL
3
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
.
ICMP Message - Echo Request (trace)
Type
8
Chk
sum
Seq.
Num
Data
DataLink
Tr.
FCS
Code
0
RTB to RTC
……
ID
UDP
(Layer 4)
DestPort
35,000
IP Header
(Layer 3)
Source IP
Add.
10.0.0.1
Dest. IP Add.
192.168.10.2
Protocol field
1
TTL
2
ICMP Message - Echo Request (trace)
Type
8
Chk
sum
ID
Seq.
Num
Data
UDP
(Layer 4)
DestPort
35,000
DataLink
Tr.
FCS
Code
0
RTC to RTD
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
……
IP Header
(Layer 3)
Source IP
Add.
10.0.0.1
Dest. IP Add.
192.168.10.2
Protocol field
1
TTL
1
ICMP Message - Echo Request (trace)
Type
8
Chk
sum
ID
Seq.
Num
Data
UDP
(Layer 4)
DestPort
35,000
DataLink
Tr.
FCS
Code
0
78
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
.2
RTD
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
DA = 192.168.10.2, TTL = 2
ICMP Time Exceeded, SA = 172.16.0.2
DA = 192.168.10.2, TTL = 3
RTB
 This time RTB decrements the TTL by 1 and it is NOT 0. (It is 2.)
 So it looks up the destination ip address in its routing table and forwards it on to the next
router.
RTC
 This time RTC decrements the TTL by 1 and it is NOT 0. (It is 1.)
 So it looks up the destination ip address in its routing table and forwards it on to the next
router.
RTD
 RTD however decrements the TTL by 1 and it is 0.
 However, RTD notices that the Destination IP Address of 192.168.0.2 is it’s own interface.
 Since it does not need to forward the packet, the TTL of 0 has no affect.
79
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
……
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
….
IP Header
(Layer 3)
Source IP
Add.
10.0.0.1
Dest. IP Add.
192.168.10.2
Protocol field
1
TTL
1
ICMP Message - Echo Request (trace)
Type
8
Chk
sum
ID
Seq.
Num
Data
UDP
(Layer 4)
DestPort
35,000
DataLink
Tr.
FCS
Code
0
IP Header
(Layer 3)
Source IP
Add.
192.168.10.2
Dest. IP Add.
10.0.0.1
Protocol field
1
ICMP Message – Port Unreachable
Type
3
Code
3
Chk
sum
ID
Seq
.
Nu
m.
Data
DataLink
Tr.
FCS
RTD
 RTD sends the packet to the UDP process.
 UDP examines the unrecognizable port number of 35,000 and sends back an
ICMP Port Unreachable message to the sender, RTA, using Type 3 and
Code 3.
80
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
.2
RTD
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
DA = 192.168.10.2, TTL = 2
ICMP Time Exceeded, SA = 172.16.0.2
DA = 192.168.10.2, TTL = 3
ICMP Port Unreachable, SA = 192.168.10.2
Data Link Header
(Layer 2)
Data Link
Data Link
Destination
Source
Address
Address
….
IP Header
(Layer 3)
Source IP
Add.
192.168.10.2
Dest. IP Add.
10.0.0.1
Protocol field
1
ICMP Message – Port Unreachable
Type
3
Code
3
Chk
sum
ID
Seq
.
Nu
m.
Data
DataLink
Tr.
FCS
Sending host, RTA
 RTA receives the ICMP Port Unreachable message.
 The traceroute program uses this information (Source IP Address) and displays
the third hop.
 The traceroute program also recognizes this Port Unreachable message as
meaning this is the destination it was tracing.
81
10.0.0.0/8
172.16.0.0/16
RTA
RTB
.1
.2
192.168.10.0/24
RTC
.1
.2
RTD
.1
.2
DA = 192.168.10.2, TTL = 1
ICMP Time Exceeded, SA = 10.0.0.2
DA = 192.168.10.2, TTL = 2
ICMP Time Exceeded, SA = 172.16.0.2
DA = 192.168.10.2, TTL = 3
ICMP Port Unreachable, SA = 192.168.10.2
Sending host, RTA
 RTA, the sending host, now displays the third hop.
 Getting the ICMP Port Unreachable message, it knows this is the final hop
and does not send any more traces (echo requests).
RTA# traceroute 192.168.10.2
Type escape sequence to abort.
Tracing the route to 192.168.10.2
1 10.0.0.2 4 msec 4 msec 4 msec
2 172.16.0.2 20 msec 16 msec 16 msec
3 192.168.10.2 16 msec 16 msec 16 msec
82
Recommended Reading
For more information on ICMP and other TCP/IP topics, I recommend:
 TCP/IP Illustrated, Volume I – R.W. Stevens
83
Chapter 6
IPv4 Addresses – Part 3
CIS 81 Networking Fundamentals
Rick Graziani
Cabrillo College
graziani@cabrillo.edu
Last Updated: 4/13/2008