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The Internet Protocol
1
Contents
• Understand the role of the Internet
Protocol (IP)
• Examine IP address classes
• Use Address Resolution Protocols (ARPs)
• Decode IP packet structure
• Examine IP on various physical networks
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The Internet Protocol
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Internetworking Concepts
•
Internet
– Different networks connected together
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Physical-Transport Independence
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• IP layer
– Provides powerful logical abstraction
– Hides Physical Layer dependency
• Upper layer processes see a logical IP network
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Functions of Internet Protocol
• Internet Protocol (IP) provides
– Datagram service
– Phisical network independence for higher layer processing
– Logical address for computers on network
– Independence from maximum transmission unit size
– Fragmentation and reassembly control
• These topics are examined in the next several viewgraphs
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IP Datagram Service
•
•
Ip makes use of “best efforts” service
– Similar to postal services
Advantages
– Simplicity and less overhead
– Upper layers can build more reliable service
– Adequate for many networks
-- LANs, frame relay
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Need for Uniform Addresses
(Logical Addresses)
•
Phisical networks use different addressing schemes
– Ethernet networks use 6-byte addresses
– X.25 networks use 14-digit decimal codes
– ARCNET networks use 1-byte addresses
•
How should nodes on a logical network be identified?
– Solution: Use a logical address to provide a uniform way of
addressing all network nodes rregardless of their physical network
connections
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Message Size Limitations
•
Physical networks support different maximum frame size
– Example: 1518 bytes for Etherne, 512 bytes for ARCNET, etc.
•
Upper layers (TCP) del with message size of arbitrary length
•
Problem: How do you send arbitrarily long messages to networks
with packet-size constraints?
– A solution: datagrams, fragmentation, and reassembly
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Names and Addresses
• LAN Addressing
– NIC addresses (like National Insurance Number)
– Broadcast technology
– No geography
• Network Addressing
– IP addresses (telephone numbers)
– Permits subnetting (like county and area codes)
– Gives routing capability
– Maps to NIC address through ARP
• Human Friendly Names
– Shows affiliations (like a normal mail address)
– Structured independently of IP
– Used to identify people, computers, networks, organistations.
– Maps to IP through DNS
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Typical MTU Size
Network
Typical frame
size (bytes)
MTU (bytes)
Maximum
frame size
(bytes)
Ethernet
1024
1500
1518
IEEE 802.5
(4 Mbps)
1024
4464
4508
IEEE 802.5
(16 Mbps)*
1024/4096
17,756
17,800
ARCNET
508
508
512
X.25
128
4080
4096
*Assuming a token-holding time of 9 ms.
MTU = maximum transmission unit
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Fragmentation and
Reassembly Control
• Many applications on hosts tend to use large message size
– File transfer, graphic applications
• Many wide area networks prefer smaller packet size in
comparison with some applications
– Better use of buffer memory
– Smaller probability of error for each packet
– Smaller delay for priority packets
• Larger packet size in networks (such as LANs) can result in
more efficient data transfer
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Freagmentation and
Reassembly Control (continued)
• What would happen to packets going from Host B to Host A?
Note: TCP messages can be fragmented by sender
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The Internet Protocol
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Calculating an Address Class
(continued)
Address Class
First Decimal Number in Dotted Decimal
Address
Minimum
Maximum
A
1
126
B
128
191
C
192
223
D
224
239
E
240
247
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Range of Assignable Addresses
netid
hostid
Class
Minimum
Maximum
Minimum
Maximum
A
1
126.
0.0.1
255.255.254
B
128.0
191.255.
0.1
255.254
C
192.0.0
223.255.255 .1
254
D
224.
239.
N/A
N/A
N/A = not applicable
•
Why is address 127.x.x.x not assigned?
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Software Loopback
•
Local machne can be addressed by 127.x.x.x
– “x” can be any value
--
Typically, 127.0.0.1 is used for local host
– Also referred to software loopback test
---
Packets never transmitted
Packets copied from transmit buffer to receive buffer
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Hostid and Broadcast Addresses
• Hostid of 0 is never assigned to an individual host
– An internet address with hostid of 0 refers to the network itself
-- Example: 144.19.0.0
-- Refers to class B network 144.19.0.0
• Directed broadcast addresses
– By convention, broadcast addresses have all 1 s in hostid field
-- Example: 144.19.255.255
– Important exception is software derived from BSD 4.2 UNIX
-- Uses all 0s broadcast
-- Example: 144.19.0.0
• Limited broadcast address
– Broadcast address of 255.255.255.255
– IP packets with this address usually do not cross router boudary
– Not all TCP/IP implementations support it
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The Internet Protocol
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The Problem: Need for Address Resolution
•
How does a host know about another host’s physical address?
– Hard code knowledge of physical addresses?
– Can you logical (IP address) to determine physical address?
-- Address resolution protocols
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Dynamic Address Resolution
Protocol Mechanism
•
Host A broadcasts ARP request on network containing B’s IP address
•
All nodes receive ARP request, but only B responds because its IP
address is included in the ARP request
•
B replies to A, with B’s physical address
•
Assumption: requires broadcast capability on network (i.e.,
Ethernet, Token Ring, etc.)
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ARP Request/Response
Packet Structure
•
Numbers in ( ) represent bits
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ARP Refinements: Caching
• In previous example
– Host A uses ARP reply to build a local cache
-- Cache contains <IP addr., Physical addr.> pair
Host B is likely to reply to A
– Use ARP request to store A’s <IP addr., Psysical addr.> in cache
• Other host extract A’s <IP addr., Physical addr> from ARP
request
• Machines booting on netork announce their <IP addr.,
Physical addr.>
– Other machines cache this information
– Also used for duplicate IP address detection
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IP Address for Disless Nodes
•
Workstations store their IP addresses in local storage media
– How do diskless workstations store their IP addresses?
– A solution: use Reverse ARP (RARP)
-- Keep <IP addr., Physical addr.? Bindings on RARP server
-- Potential for simplifying IP adress administration?
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RARP Operation
Sender address – PA
Destination address = broadcast
Ethertype = 8035 hex
• RARP uses same packet structure as ARP
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RARP Operation (continued)
• Send broadcasts RARP request
– SENDER HA ← Sender’s physical address
– TARGET HA ← Sender’s physical address
• RARP servers respond with
– OPERATION TYPE ← reply
– TARGET IP ← Answer (requester’s IP address)
– DATA LINK DA ← Requester’s physical address
• Lssues
– RARP request storms
– Primary and backup RARP servers
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BOOTP
• BOOTP makes use of UDP/IP to obtain IP addresses and
other information
• BOOTP does not provide clients with bootstrap image
– It provides the name of the boot image
– Boot image is transferred using Trivial File Transfer Protocol
(TFTP)
• To forward BOOTP requests across routers, routers must be
configured with rekay agents to foeward BOOTP packets
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Troubleshooting Duplicate IP Addresses
and ARP Tables
• Nodes on an IP network must have unique IP
addresses
– Otherwise, ARP tables are initialized with incorrect <IP addr., Physical
addr.> mappings
– Symptoms of bad ARP tables are
-- Users unable to access TCP/IP hosts
-- Workstations and servers crashing
-- Intermittent problems with applications not working
• Common results of duplicate IP addresses are
– ARP table corruption at workstations
– ARP table corruption at servers
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Duplicate IP Addresses at Workstation
•
Step 1
– Workstation initiates FTP session to server
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Duplicate IP Addresses at Workstation (continued)
•
Step 2
– Second workstation with duplicate IP address initiates FTP session to server
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Duplicate IP Addresses at Workstation
(continued)
• If the server receives a TCP/IP connection request from a
second workstation with a duplicate IP address, the TCP/IP
software may
– Ignore the second request
– Overwrite the server ARP cache entry with hardware address from
second workstation
– Get confused and crash
• In either of the above choices, one or both of the workstations
with the duplicate IP address will have connection problems
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Duplicate IP Addresses at the Server
•
•
•
Workstation tries to connect to
VAX at IP address 144.19.74.102
If the SUN server at duplicate IP
address 144.19.74.102 returns an
ARP reply faster than the VAX, the
workstation connects to the SUN
server instead of the VAX
What happens if the SUN server
and VAX server also act as
routers?
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ARP Display Utilities
• Resolving duplicate IP address problems can be a challenging
task on large networks
– Keeping good records of IP address assignments and hardware
addresses of devices can help
– Use utilities to display and fix ARP cache entries
-- Most UNIX systems have the arp utility
arp -a
arp -d
arp -s
hostname
hostname hardware_addr
Display all ARP entries in table
Delete an entry from ARP table
Add a new entry in ARP table. Entry is not
timed out!
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The Internet Protocol
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Hands-On Exercise 3.1: Address Resolution
•
Your instructor will guide you to Hands-On Exercise 3.1 in the
Exercise Manual
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The Internet Protocol
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Hands-On Exercise 3.2: Observing Effects of
Duplicate IP Addresses
•
Your instructor will guide you to Hands-On Exercise 3.2 in the
Exercise Manual
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The Internet Protocol
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IP Packet Structure
Background:
This exercise is a guided tour on the structure of IP packets. It will be done
concurrently with the lecture, which will explain the IP structure. You will use
the packet trace that you saved in an earlier exercise for understanding the
IP packet structure.
Objectives:
• Examine the IP packet fields
• Understand the functionality of the IP protocol
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IP Packet Structure (continued)
1.
Run LANWatch at your workstation.
If you forgot how to run LANWatch, see page 16 in the
Exercise Manual.
2.
Load the filt TELNET. TR1 that contains the TELNET packet trace
youstored in an earlier exercise.
3.
Highlight one of the red packets that contain IP protocol information
and display it in the detailed format.
Follow the instructions given to you by the instructor.
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IP Field: Version
•
Version field
– Indicates format of IP header
– Declares version of protocol to which datagram belongs
– Allows development of new protocols while network is operational
•
What is the version of the IP packet on your screen?
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IP Field: Internet Header Length
•
•
•
Internet header length
– Measured in 32-bit words
– Required because IP header contains variable length options field
What is the internet header length of the IP packet on your
screen?___________________________
Does the IP packet have an options field?
Yes
No
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IP Field: Type of Service (TOS)
•
Type of service
– Informs networks on Quality Of Service (QOS) desired
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IP Field: Type of Service (TOS)
•
•
What is the bit pattern for TOS of IP packet on your screen?
__________________________
What is the TOS value? _________________
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IP Field: Total Length
•
Total length
–
–
•
All hosts must be prepared to receive datagrams of 576 octets
–
•
Length of datagram (octets), including IP header and data portion
Maximum datagram size is 65,535 octets
512 octets of data and 64 octets of protocol overhead
What is the total length for the IP packet on your screen? _________________ octets?
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IP Field: Identification
•
Identification
– Set uniquely for each datagram
– Used as an aid in assembling fragments of a datagram
•
What is the identification value for the IP packet on your screen? ___________
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IP Field: Identification (continued)
•
Use cursor keys (↑, ↓) to examine identification field values of IP packets before
and after this IP packet
– What is the identification value of the previous IP packet?______
– What is the identification value of the next IP packet?________
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IP Field: Flags
•
What are the flag settings for the IP packet on your screen?
DF flag =_______ MF flag = _______
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IP Field: Fragment Offset
•
•
•
Fragment offset
– Position of fragment's data relative to the beginning of data carried in original datagram
Maximum of 8192 fragments per datagram
Identification field is same for all fragments
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IP Field: Fragment Offset
•
What is the fragment offset for the IP packet on your
screen?_________________________
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IP Field: Time to Live
•
Time to live
– Maximum time IP datagram can remain on internet
– When TTL = 0, IP datagram is destroyed (dropped)
– Decreased by time for IP header processing, but must be decreased by at
least 1
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•
What is the TTL field value for the IP datagram on your
screen?____________________
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IP Field: Protocol
•
•
Protocol field
– Indicates which Upper Layer Protocol (ULP) is to receive data portion of IP datagram
What is the protocol field value for the IP packet on your screen?
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IP Field: Protocol (continued)
Protocol field value
Keyboard
0
Description
Reserved
1
ICMP
Internet Control Message
Protocol
6
TCP
Transmission Control
Protocol
8
EGP
Exterior Gateway Protocol
9
IGP
Any private Interior
Gateway Protocol
11
NVP
Network Voice Protocol
17
UDP
User Datagram Protocol
22
XNS IDP
Xerox Network System’s
Internet Datagram Protocol
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ISO TP4
ISO Transport Protocol
class 4
89
OSPF
Open shortest path first
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IP Field: Header Checksum
•
Header checksum
– Covers only the IP header
– Add up 1's complement of each data item (16-bit) and then the 1 's complement of the
sum
– Recomputed at every route because TTL field changes
•
What is the header checksum field value of the IP packet?_______________________
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IP Field: Source Address, Destination Address
•
•
Source and destination addresses are divided in netid and hostid fields
What are the source-address and destination-address fields of the IP packet on your
screen?
Source address: _______________
Destination address: ____________
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IP Field: Options
•
Options
– Officially defined options are
-- Security, loose source routing
-- Strict source routing, record route
-- Stream ID, Internet timestamp
•
Options are of two types
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•
Are there any options defined in the IP packet on your screen?_________________________
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Bonus
•
Pick an IP packet (that appeals to you!) within LANWatch and analyze it on your own.
Try to identify the blank fields in the IP datagram above. Label these fields and enter the
value in the fields for the IP packet you are analyzing.
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The Internet Protocol
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Duplicate IP Address Problem
•
IP addresses must be unique
– Most network software assumes trusted hosts
•
Duplicate IP addresses result in
– Network software becoming confused, malfunctioning
– Routing problems
-- Because routing information is encoded in IP address netid and hostid
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Buffer Reassembly Problem
•
Not all IP implementations are equally robust
–
Some IP implementations may not reassemble datagram fragments
correctly
–
Solution: Configure IP software for DF = 1
-- Problem:
1. DF flag may not be configurable by network manager
2. Fragmentation may be required if IP datagram traverses networks
with small MTU
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IP Trailers: 4BSD UNIX
•
•
Software derived from BSD 4.2 UNIX may use alternate IP encapsulation
– Done for efficient memory management
-- To place data information on page boundary
Berkeley-style trailer encapsulation
– Will not interoperate with normal IP encapsulation (example: IP routers)
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Avoiding IP Trailers
•
•
On many UNIX systems, IP trailer encapsulation can be controlled by the if conf
ig utility
Example:
ifconfig ethO -trailers
ifconfig ethO
ethO: flags=23<UP,BROADCAST,NOTRAILERS>
inet 144.19.74.201 netmask ffffOOOO broadcast 144.19.255.255
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All O's Broadcast
•
IP software implemented on earlier BSD 4.2 UNIX may use all O's broadcast
– Can cause confusion with most systems that use all 1's broadcast
•
Use if conf ig utility to enable all 1's broadcast
– May not work if broadcast mechanism has been hard-coded
•
Example:
ifconfig ethO broadcast 144.19.255.255
ifconfig ethO
ethO: flags=23<UP,BROADCAST,NOTRAILERS>
inet 144.19.74.201 netmask ffffOOOO broadcast 144.19.255.255
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The Internet Protocol
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Using Unique Internet Addresses
•
•
•
If building your own private internet
– Decide on an IP address class
-- Popular choices are class B, class C
If you decide not to connect to the Internet
– You can select your own IP network number
If you decide to connect to the Internet
– You should apply to Network information Center (NIC) for unique internetwork
number
– Alternatively, use IP address translation devices such as application-level
gateways
-- Application-level gateways also can be used to implement
-- firewalls for enhancing security
-- Can be used to avoid duplicate IP address conflicts
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Application-Level Security Gateway (Firewall)
•
Application-level firewall provides
– Isolation between duplicate IP addresses
– Security by restricting access between internal and external networks at the Application
Layer
– Alternatively, use a private address
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Obtaining Unique IP Network Number
• To obtain Internet number to connect to the Internet, apply to
– Network Solutions
InterNIC Registration Services
505 Huntmar Park Drive
Herndon, VA 22070 USA
[email protected]
• Users wanting to connect to the MILNET must still apply to
– DDN Network Information Center
14200 Park Meadow Drive, Suite 200
Chantilly, VA 22021 USA
[email protected]
• See Appendix D for application form for Obtaining IP
Network Number
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IP Address Database
• Local management of IP addresse
– Configuration database kept on local machine
-- Often simple text files, such as /etc/hosts (UNIX) or
net . cfg and config . tel, etc. (MS-DOS)
–
–
–
–
RARP servers
BOOTP server
DHCP server (covered in Course 154)
Name servers
• RARP servers can
– Simplify IP address maintenance
– Problems:
-- Updates when data-link address changes
-- RARP storms
-- Single point of failure
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The Internet Protocol
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IP on IEEE 802 LANs
• Initial IP implementation on LANs was on Ethernet
– Ethertype field in Ethernet header is used to indicate IP packet
-- Ethertype = 800 hex for IP packets
•
There is no Ethertype field in IEEE LANs
- How do you indicate Ethertype information?
-- Use lEEE802.2LLC
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IEEE 802.2 Logical Link Control
•
IEEE LANs use a sublayer called LLC to indicate protocol (software)
addresses
74
The SNAP Protocol
•
•
A special DSAP or SSAP value in IEEE 802.2 field indicates that Ethertype field
is in the data field
This mechanism is called SubNet Access Erotocol (SNAP)
75
IP on IEEE 802.3, 802.5, and FDDI
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IP on X.25
• First octet in X.25 call request data field indicates IP protocol
– X.25 virtual circuit is used to transmit datagrams
-- Closed after a period of inactivity
-- Treated as a point-to-point circuit
77
IP on ATM
–
–
ATM transmits data in fixed
53-byte cells (5 bytes
header and 48 bytes data)
ATM cells available on
demand with low latency
-- Real-time audio/video
-- Multimedia applications
–
ATM provides virtual
channels with quality of
service (QoS) parameters
78
Chapter Summary
You have learned about
•
The role of the Internet Protocol (IP)
•
IP address classes
•
Address Resolution Protocols
•
IP packet structure
•
IP on various physical networks
79