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2. 2
C/Embedded Base Camp
Networking essentials.
Lecture 2
Petro Shevchenko
Maksym Vysochinenko
October 2021

3. 3
1. Network Layer (L3)
2. IPv4 Packets
3. IPv4 Addresses
4. Network Address Translation (NAT)
5. IPv6 Packets
6. IPv6 Addresses
7. Introduction to Routing
8. ICMP Messages
Agenda

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Network Layer (L3)

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Network Layer Characteristics
The Network Layer
• Provides services to allow end devices to exchange data
• IP version 4 (IPv4) and IP version 6 (IPv6) are the principle
network layer communication protocols.
• The network layer performs four basic operations:
• Addressing end devices
• Encapsulation
• Routing
• De-encapsulation

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Network Layer Characteristics
IP Encapsulation
• IP encapsulates the transport layer
segment.
• IP can use either an IPv4 or IPv6 packet
and not impact the layer 4 segment.
• IP packet will be examined by all layer 3
devices as it traverses the network.
• The IP addressing does not change from
source to destination.
Note: NAT will change addressing, but will
be discussed in a later module.

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Network Layer Characteristics
Best Effort
IP is a “Best Effort” protocol:
• IP will not guarantee delivery of the packet.
• IP has reduced overhead since there is no
mechanism to resend data that is not received.
• IP does not expect acknowledgments.
• IP does not know if the other device is operational
or if it received the packet.
IP is unreliable:
• It cannot manage or fix undelivered or corrupted
packets.
• IP cannot retransmit after an error.
• IP cannot realign out of sequence packets.
• IP must rely on other protocols for these functions.

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Network Layer Characteristics
Media Independent
IP is media Independent:
– IP does not concern itself with the type of
frame required at the data link layer or the
media type at the physical layer.
– IP can be sent over any media type:
copper, fiber, or wireless.
Fragmentation is when Layer 3 splits the IPv4
packet into smaller units.
– Fragmenting causes latency.
– IPv6 does not fragment packets.
– Example: Router goes from Ethernet to a
slow WAN with a smaller MTU
The network layer will establish the Maximum
Transmission Unit (MTU).
– Network layer receives this from control
information sent by the data link layer.
– The network then establishes the MTU size.

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IPv4 Packets

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IPv4 Packet
IPv4 Packet Header Fields (RFC 791)
Significant fields in the IPv4 header:
● Version - protocol version (4 or 6) (4 bits)
● IHL - IP header length in 32 bit words (4 bits)
● Type of service - used for QoS (8 bits)
● Total Length - includes IP header and data in bytes
(16 bits)
● Identification - used for packet fragmentation (16
bits)
● Flags - used for packet fragmentation (3 bits)
● Fragment Offset - used for packet fragmentation
(13 bits)
● Time to Live - maximum datagram lifetime (8 bits)
● Protocol - the next level protocol (8 bits)
● Header Checksum (16 bits)
● Source Address - source IP address (32 bits)
● Destination Address - destination IP address (32
bits)
● Options - may appear or not in datagrams (variable
length)

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IPv4 Packet
IPv4 Packet fragmentation
Fragmentation fields in the IPv4 header:
● Identification - assigned by the sender to
aid in assembling the fragments of a
datagram (16 bits)
● Flags (0, 1, 2) (3 bits)
○ Bit 0: reserved, must be zero
○ Bit 1: (DF) 0 = May Fragment, 1 =
Don't Fragment.
○ Bit 2: (MF) 0 = Last Fragment, 1 =
More Fragments.
● Fragment Offset - indicates where in the
datagram this fragment belongs in 64 -bit
words (13 bits)

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IPv4 Packet
IPv4 Maximum Transaction Unit
Limitations of encapsulation :
● HW MTU - hardware maximum
transaction unit (1514 bytes for Ethernet)
● IP MTU - IP maximum transaction unit
(1500 bytes for Ethernet)
● TCP MSS - TCP maximum segment size
(1460 bytes for Ethernet)

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IPv4 Packets
Limitations of IPv4
IPv4 has three major limitations:
– IPv4 address depletion – We have basically run out of IPv4 addressing.
– Lack of end-to-end connectivity – To make IPv4 survive this long, private addressing and NAT were
created. This ended direct communications with public addressing.
– Increased network complexity – NAT was meant as temporary solution and creates issues on the
network as a side effect of manipulating the network headers addressing. NAT causes latency and
troubleshooting issues.

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IPv4 Addresses

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IPv4 Address Structure
Network, Host, and Broadcast Addresses
• Within each network are three types of IP addresses:
• Network address
• Host addresses
• Broadcast address
Network Portion Host Portion Host Bits
Subnet mask
255.255.255.0 or /24
255 255 255
11111111 11111111 11111111
0
00000000
Network address
192.168.10.0 or /24
192 168 10
11000000 10100000 00001010
0
00000000
All 0s
First address
192.168.10.1 or /24
192 168 10
11000000 10100000 00001010
1
00000001
All 0s and a 1
Last address
192.168.10.254 or /24
192 168 10
11000000 10100000 00001010
254
11111110
All 1s and a 0
Broadcast address
192.168.10.255 or /24
192 168 10
11000000 10100000 00001010
255
11111111
All 1s

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IPv4 Unicast, Broadcast, and Multicast
Unicast
• Unicast transmission is sending a packet to one destination IP address.
• For example, the PC at 172.16.4.1 sends a unicast packet to the printer at 172.16.4.253.

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IPv4 Unicast, Broadcast, and Multicast
Broadcast
• Broadcast transmission is sending a packet to all other destination IP addresses.
• For example, the PC at 172.16.4.1 sends a broadcast packet to all IPv4 hosts.

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IPv4 Unicast, Broadcast, and Multicast
Multicast
• Multicast transmission is sending a packet to a multicast address group.
• For example, the PC at 172.16.4.1 sends a multicast packet to the multicast group address
224.10.10.5.

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Types of IPv4 Addresses
Public and Private IPv4 Addresses
• As defined in in RFC 1918, public IPv4 addresses are globally routed between internet service
provider (ISP) routers.
• However, private addresses are not globally routable.
• Private addresses are common blocks of
addresses used by most organizations to assign
IPv4 addresses to internal hosts.
• Private IPv4 addresses are not unique and can
be used internally within any network.
Network
Address and
Prefix
RFC 1918 Private Address
Range
10.0.0.0/8 10.0.0.0 - 10.255.255.255
172.16.0.0/12 172.16.0.0 - 172.31.255.255
192.168.0.0/16 192.168.0.0 - 192.168.255.255

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Network Address Translation (NAT)

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IPv4 Packets
Network Address Translation (NAT)
What is NAT :
● NAT is a method of mapping an IP
address space into another by modifying
network address information in the IP
header
● The purpose of NAT creation is to
overcome the shortage of IP addresses
available on the Internet
● There are two types of IPv4 addresses:
○ Private IP addresses (RFC 1918):
10.0.0.0/8, 172.16.0.0/12,
192.168.0.0/16
○ Public IP addresses are IP
addresses that are used on the
Internet. They are allocated by IANA

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IPv4 Packets
Network Address Translation (NAT)
There are three types of NAT:
● Static NAT
● Dynamic NAT
● IP masquerading
NAT requires a NAT Translation Table

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IPv6 Packets

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IPv6 Packets
IPv6 Overview
• IPv6 was developed by Internet
Engineering Task Force (IETF).
• IPv6 overcomes the limitations of IPv4.
• Improvements that IPv6 provides:
• Increased address space – based on 128
bit address, not 32 bits
• Improved packet handling – simplified
header with fewer fields
• Eliminates the need for NAT – since there
is a huge amount of addressing, there is
no need to use private addressing
internally and be mapped to a shared
public address

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IPv6 packets
IPv4 vs IPv6 addressing

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IPv6 Packets
IPv6 Packet Header Fields (RFC 2460)
Significant fields in the IPv6 header :
● Version number (6) (4 bits)
● Traffic Class - used for QoS (8 bits)
● Flow Label - Informs device to handle
identical flow labels the same way (20 bits)
● Payload length - indicates the length of
the data portion of the IPv6 packet (16
bits)
● Next Header - identifier of next level
protocol: ICMP, TCP, UDP, etc.
● Hop Limit - Replaces TTL field Layer 3
hop count (8 bits)
● Source Address - source IPv6 address
(128 bits)
● Destination Address - destination IPv6
address (128 bits)

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IPv6 Packets
IPv6 Path MTU Discovery
1. The source host sends a packet no larger than its MTU to
the destination host.
2. If the MTU of a device's output interface is smaller than the
packet, the device performs the following operations
● Discards the packet.
● Returns an ICMPv6 error message containing the
interface MTU to the source host.
1. Upon receiving the ICMPv6 error message, the source host
performs the following operations:
● Uses the returned MTU to limit the packet size.
● Performs fragmentation.
● Sends the fragments to the destination host.
1. Step 2 and step 3 are repeated until the destination host
receives the packet. In this way, the source host finds the
minimum MTU of all links in the path to the destination host.

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IPv6 Addresses

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IPv6 Addresses
IPv6 Addresses Scope
● Global Unicast Addresses (starts at 0x2 or 0x3)
○ Operate on the Internet
○ Allocated by IANA
● Unique Local Addresses (starts at 0xFD)
○ not routed on the Internet
○ used without IANA permissions
● Link-local Addresses (starts at 0xFE80)
○ not routed
○ assigned automatically
Unique Local Address:
L = 1 the prefix is locally assigned
L = 0 for future use

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IPv6 Addresses
Special IPv6 Addresses
There are special IPv6 addresses:
● Current host ::/128
● Default route ::/0
● Loopback ::1/128
● All hosts in the communication channel FF02::1
● All routers in the communication channel FF02::2
A host can create link-local address from MAC address:
● Insert 0xFFFE in the middle of IPv6 address
● Use vendor code of MAC address at the left side
● Use unique number of MAC address at the right
side
● Invert Local Administered flag

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IPv6 Address Types
Unicast, Multicast, Anycast
There are three broad categories of IPv6 addresses:
• Unicast – Unicast uniquely identifies an interface on an IPv6-enabled device.
• Multicast – Multicast is used to send a single IPv6 packet to multiple destinations.
• Anycast – This is any IPv6 unicast address that can be assigned to multiple devices. A packet sent to an anycast address is routed to the nearest device
having that address.
Note: Unlike IPv4, IPv6 does not have a broadcast address. However, there is an IPv6 all-nodes multicast address that essentially gives the same result.

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Dynamic Addressing for IPv6 GUAs
RS and RA Messages
Devices obtain GUA(Global Unicast Addresses) addresses dynamically through Internet Control
Message Protocol version 6 (ICMPv6) messages.
– Router Solicitation (RS) messages are sent by host devices to discover IPv6 routers
– Router Advertisement (RA) messages are sent by routers to inform hosts on how to obtain an IPv6 GUA and provide useful
network information such as:
• Network prefix and prefix length
• Default gateway address
• DNS addresses and domain name
– The RA can provide three methods for configuring an IPv6 GUA :
• SLAAC - Stateless Address Autoconfiguration
• SLAAC with stateless DHCPv6 server
• Stateful DHCPv6 (no SLAAC)

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Introduction to Routing

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Introduction to Routing
Dual stack concept
IPv6 tunneling
Proxying and translation (NAT-PT)
Independent communication

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Introduction to Routing
Host Routing Tables
• On Windows, route print or
netstat -r to display the PC
routing table
• Three sections displayed by
these two commands:
– Interface List – all
potential interfaces and
MAC addressing
– IPv4 Routing Table
– IPv6 Routing Table

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Introduction to Routing
IP Router Routing Table
There three types of routes in a router’s routing table:
• Directly Connected – These routes are automatically added by the router, provided the interface is
active and has addressing.
• Remote – These are the routes the router does not have a direct connection and may be learned:
• Manually – with a static route
• Dynamically – by using a routing protocol to have the routers share their information with each other
• Default Route – this forwards all traffic to a specific direction when there is not a match in the routing
table

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Introduction to Routing
Static Routing
Static Route Characteristics:
• Must be configured manually
• Must be adjusted manually by the
administrator when there is a change in the
topology
• Good for small non-redundant networks
• Often used in conjunction with a dynamic
routing protocol for configuring a default
route

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Introduction to Routing
Dynamic Routing
Dynamic Routes Automatically:
• Discover remote networks
• Maintain up-to-date information
• Choose the best path to the destination
• Find new best paths when there is a
topology change
Dynamic routing can also share static default
routes with the other routers.
Commonly used protocols – EIGRP, OSPF,
BGP.

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ICMP Messages

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ICMP Messages
ICMPv4 and ICMPv6 Messages
• Internet Control Message Protocol (ICMP) provides feedback about issues related to the processing of IP
packets under certain conditions.
• ICMPv4 is the messaging protocol for IPv4. ICMPv6 is the messaging protocol for IPv6 and includes
additional functionality.
• The ICMP messages common to both ICMPv4 and ICMPv6 include:
• Host reachability
• Destination or Service Unreachable
• Time exceeded
Note: ICMPv4 messages are not required and are often not allowed within a network for security reasons.

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ICMP Messages
Host Reachability
ICMP Echo Message can be used to test the
reachability of a host on an IP network.
In the example:
• The local host sends an ICMP Echo
Request to a host.
• If the host is available, the destination
host responds with an Echo Reply.

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Ping and Traceroute Tests
Ping – Test Connectivity
• The ping command is an IPv4 and IPv6 testing
utility that uses ICMP echo request and echo reply
messages to test connectivity between hosts and
provides a summary that includes the success rate
and average round-trip time to the destination.
• If a reply is not received within the timeout, ping
provides a message indicating that a response was
not received.
• It is common for the first ping to timeout if address
resolution (ARP or ND) needs to be performed
before sending the ICMP Echo Request.

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Ping and Traceroute Tests
Ping the Loopback
Ping can be used to test the internal
configuration of IPv4 or IPv6 on the local host. To
do this, ping the local loopback address of
127.0.0.1 for IPv4 (::1 for IPv6).
• A response from 127.0.0.1 for IPv4, or ::1 for
IPv6, indicates that IP is properly installed on
the host.
• An error message indicates that TCP/IP is not
operational on the host.

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Ping and Traceroute Tests
Ping a Remote Host
Ping can also be used to test the ability of a local
host to communicate across an internetwork.
A local host can ping a host on a remote network.
A successful ping across the internetwork confirms
communication on the local network.
Note: Many network administrators limit or prohibit the
entry of ICMP messages therefore, the lack of
a ping response could be due to security restrictions.

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Ping and Traceroute Tests
Traceroute – Test the Path
• Traceroute (tracert) is a utility that is used to test
the path between two hosts and provide a list of
hops that were successfully reached along that
path.
• Traceroute provides round-trip time for each hop
along the path and indicates if a hop fails to
respond. An asterisk (*) is used to indicate a lost
or unreplied packet.
• This information can be used to locate a
problematic router in the path or may indicate
that the router is configured not to reply.
Note: Traceroute makes use of a function of the TTL field
in IPv4 and the Hop Limit field in IPv6 in the Layer 3
headers, along with the ICMP Time Exceeded message.

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Ping and Traceroute Tests
Traceroute – Test the Path (Cont.)
• The first message sent from traceroute will have a
TTL field value of 1. This causes the TTL to time out
at the first router. This router then responds with a
ICMPv4 Time Exceeded message.
• Traceroute then progressively increments the TTL
field (2, 3, 4...) for each sequence of messages. This
provides the trace with the address of each hop as
the packets time out further down the path.
• The TTL field continues to be increased until the
destination is reached, or it is incremented to a
predefined maximum.

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Thank You